When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I have a 1986 convertible C4 with L98 engine.
It starts well and runs fine.
After a drive when the engine is hot (With normal temperature) if I turn off the engine and try to start again after few minutes (no immediate) The engine does not hold idle and if I try to hold the gas pedal it will run with bad gas mixture and will not get to high RPM
If I'll drive like this for few minutes - it will eventually be solved and the engine will work fine again or alternatively if I'll wait for 20 min or so and start - it will run OK.
I replaced the IAC and few heat sensors (front intake manifold, Radiator, and passenger side head) and also replaces the connectors with new ones - it did not solve the problem.
Probably one or more injectors are leaking, getting fuel into the cylinders when not running.
I had same issue on my '85, replaced injectors solved the same issu.
Probably one or more injectors are leaking, getting fuel into the cylinders when not running.
I had same issue on my '85, replaced injectors solved the same issu.
How come it takes few minutes of driving with a high fuel mixture and no RPMs until it is solved? can take even 10 plus minutes ... ?
and will die on idle if I leave the gas ...
Welcome to the Corvette Forum! This is the place for answers to complicated Corvette issues. Or at least we try really hard to help our fellow Corvette owners!
I would take a look at your fuel injectors as your description of what happens sounds like they are going bad. I replaced the injectors on my engine and learned that one had been dripping a lot of fuel which was the primary cause of not starting easily when hot. I would try and attach a fuel pressure gauge onto the fuel rail and watch the fuel pressure after shutdown. It should hold some pressure for a while, if it goes down to zero fast it is likely that you have a bad injector. You might measure the resistance of each of the eight injectors, the values should all be close.
On the L98 there is a Coolant Temperature Sensor and it is located up on the plenum in front. The factory service manuals will show you where it is and have a chart showing resistance and the corresponding temperature. The CTS is very important to the air/fuel ratios being calculated by the ECM. The CTS is the sensor that when it fails can makes hard starting when Hot or Cold. Take a resistance measurement from the current CTS and compare it to the chart to se what temperature it thinks it is. If the sensor thinks it is cooler it might try and add fuel to help it start. In this mode it is like a bad Choke that is staying on longer than it should. If the CTS thinks it is hotter than it really is then it will cut fuel and be very hard to start when cold.
When I re-read your post it seems very likely that it is the injectors as if it was the CTS it would clear up and run fine after time. Check your Fuel pressure regulator to be sure it is okay, they have a diaphragm that is susceptible to Ethanol which is used in our gasoline over here. Something tells me that you don't use gasoline with ethanol in it in Israel. The easy way to know if the FPR diaphragm is okay is to remove it's vacuum line and smell inside of it. If you smell raw gasoline then you should replace the FPR diaphragm. Your is probably okay but it is worth checking.
I would pull your injectors and replace them with newer better quality injectors, I have 8 Bosch Injectors that I replaced years ago and they are tolerant of ethanol. I hope the prices are not too high in Israel for injectors. I paid less than $200 for the set of eight brand new Bosch injectors. If you have trouble finding them I would be happy to mention a couple U.S. located Injector sales groups.
Once you replace the injectors I would bet your Corvette will be burning up the roads of Israel once again! I am envious as that must be a fun place to own a Corvette!
I spent years in the Middle East so I am familiar with the terrain. I was near Israel back during the Cuban Missile Crisis (October,1962). The Border guards would not let a U.S. Diplomat who drove south from Turkey under orders to get to Israel into Israel. Since we couldn't get into Israel so we went back to Beirut and spent two weeks at the beach at a nice French hotel. I have several friends who are Israeli and one day I want to come visit them. How much do you pay for gasoline over there?
Be sure to keep the EGR functioning and you will get along with cheaper gas, when the EGR fails then the cylinder combustion temperature goes up and starts creating Nox. A bad EGR will force the owner to use a higher octane fuel to avoid pinging or detonation. I hear of people who have to use premium fuel in their Corvette and I can predict that their EGR is dead.
Another good test is to be sure that your Knock Sensor is working, take a wrench or something metallic and tap the block on a head or someplace metal, not too hard. When you do this the idle should change briefly as it retards the ignition and then return to normal. I had a bad knock sensor that was retarding the timing all the time and my Corvette felt like a Renault four cylinder. With a new knock sensor my car was totally different to drive. It was a Corvette again!
I made an assumption that you would have a set of the Factory Service Manuals. If I am wrong then you should try and get one. The hard copies are about $85 and then they offer them on DVD's from RockAuto for less than $30.
You NEED to have the FSM's for your Corvette if you plan on owning it long. They are priceless to have when working on a C4 as they have trouble shooting charts and tons of information about the C4's.
Welcome to the Corvette Forum! This is the place for answers to complicated Corvette issues. Or at least we try really hard to help our fellow Corvette owners!
I would take a look at your fuel injectors as your description of what happens sounds like they are going bad. I replaced the injectors on my engine and learned that one had been dripping a lot of fuel which was the primary cause of not starting easily when hot. I would try and attach a fuel pressure gauge onto the fuel rail and watch the fuel pressure after shutdown. It should hold some pressure for a while, if it goes down to zero fast it is likely that you have a bad injector. You might measure the resistance of each of the eight injectors, the values should all be close.
On the L98 there is a Coolant Temperature Sensor and it is located up on the plenum in front. The factory service manuals will show you where it is and have a chart showing resistance and the corresponding temperature. The CTS is very important to the air/fuel ratios being calculated by the ECM. The CTS is the sensor that when it fails can makes hard starting when Hot or Cold. Take a resistance measurement from the current CTS and compare it to the chart to se what temperature it thinks it is. If the sensor thinks it is cooler it might try and add fuel to help it start. In this mode it is like a bad Choke that is staying on longer than it should. If the CTS thinks it is hotter than it really is then it will cut fuel and be very hard to start when cold.
When I re-read your post it seems very likely that it is the injectors as if it was the CTS it would clear up and run fine after time. Check your Fuel pressure regulator to be sure it is okay, they have a diaphragm that is susceptible to Ethanol which is used in our gasoline over here. Something tells me that you don't use gasoline with ethanol in it in Israel. The easy way to know if the FPR diaphragm is okay is to remove it's vacuum line and smell inside of it. If you smell raw gasoline then you should replace the FPR diaphragm. Your is probably okay but it is worth checking.
I would pull your injectors and replace them with newer better quality injectors, I have 8 Bosch Injectors that I replaced years ago and they are tolerant of ethanol. I hope the prices are not too high in Israel for injectors. I paid less than $200 for the set of eight brand new Bosch injectors. If you have trouble finding them I would be happy to mention a couple U.S. located Injector sales groups.
Once you replace the injectors I would bet your Corvette will be burning up the roads of Israel once again! I am envious as that must be a fun place to own a Corvette!
I spent years in the Middle East so I am familiar with the terrain. I was near Israel back during the Cuban Missile Crisis (October,1962). The Border guards would not let a U.S. Diplomat who drove south from Turkey under orders to get to Israel into Israel. Since we couldn't get into Israel so we went back to Beirut and spent two weeks at the beach at a nice French hotel. I have several friends who are Israeli and one day I want to come visit them. How much do you pay for gasoline over there?
Be sure to keep the EGR functioning and you will get along with cheaper gas, when the EGR fails then the cylinder combustion temperature goes up and starts creating Nox. A bad EGR will force the owner to use a higher octane fuel to avoid pinging or detonation. I hear of people who have to use premium fuel in their Corvette and I can predict that their EGR is dead.
Another good test is to be sure that your Knock Sensor is working, take a wrench or something metallic and tap the block on a head or someplace metal, not too hard. When you do this the idle should change briefly as it retards the ignition and then return to normal. I had a bad knock sensor that was retarding the timing all the time and my Corvette felt like a Renault four cylinder. With a new knock sensor my car was totally different to drive. It was a Corvette again!
I made an assumption that you would have a set of the Factory Service Manuals. If I am wrong then you should try and get one. The hard copies are about $85 and then they offer them on DVD's from RockAuto for less than $30.
You NEED to have the FSM's for your Corvette if you plan on owning it long. They are priceless to have when working on a C4 as they have trouble shooting charts and tons of information about the C4's.
Best regards,
Chris
Chris,
Thanks for the very detailed response.
I am not sure I understand what plenum is.
I do have a few questions,
When I turn of the engine after a drive and start it shortly after again it will run fine, so I am a bit confused how come the issue happens only if I wait a bit longer and will not happen if I wait a bit longer like 30 min.
I found many injectors on Amazon but not Bosh ones. Are they any good?
Thank you for the very detailed answer above,
I do have some questions:
1. I understand the Injectors may have an issue - but how come when I turn off the engine for a short time (a minute for example) and start it again it will run fine? Also if I wait a little longer like half an hour?
2. I do not know what plenum is, I did replace the sensor in the front of the intake manifold.
I did not find Bosh injectors but did find these on Amazon:
1. In the short time the engine is "Off" the injector leaking fuel has not puddled enough fuel to make it hard to start. If you wait 1/2 hour the hot engine might have made the fuel evaporate and let it start easily again. Even a cool engine will evaporate gasoline pretty quickly. On my C4 it would get very hard to start when hot as it was way too rich with all the excess fuel from my leaking injector. Once the pressure drops in the fuel system it should potentially stop dripping and wait for the next time it gets pressurized and start leaking out the nozzle again. Whenever there is any residual pressure in your fuel system the injector will be leaking most likely.
When you shut the engine off and start it again soon after it should not have leaked enough to make a problem. If the engine is flooded you push the gas pedal down to 80% and crank the engine to clear it all out. You might try that next time it hesitates to start.
2. I apologize for that, the "Plenum" is the large silver painted Aluminum structure above the intake manifold connected to the engine by the "Runners" on each side of the engine. It is a part of the intake manifold behind the throttle body itself.
I bought my injectors from a company near New York City and they sold me the Bosch brand as the shop owner suggested they were the "best" out there. The LOW price of the Amazon injectors would scare me away, I would rather pay a little more for "quality" parts. The company near New York is called South Bay Injectors and there is another company called FIC and a lot of folks on the Forum use them exclusively. Try and find some Bosch injectors as they must use them in Israel as well. I don't like replacing parts I put on the Corvette so I bought the $170 set and did it myself. I looked at the SBI website and they show a set of 24 pound Bosch D3 Injectors for $180 a set. They are a lot more expensive but the quality is there. I would almost bet money that the ones on Amazon are from China and typically junk.
Before spending a lot of money it might be worth it to find someone in Israel who rebuilds injectors and have them clean the ones you have. The original injectors were made by Multitech (sp?) and they do not last when exposed to ethanol. I think they are a dark grey but they must say someplace on the injector. The cost of cleaning the injectors might be less than the cost of new ones. Here in the United States most Fuel Injector shops will test and clean them for you before selling you a new set. If you still have the original injectors then you might want to replace them outright as they were not of the best quality, I don't know why GM chose them.
I have not mentioned this but I thought I would. The Oxygen sensor is designed to last 24 months or 50 k miles, once they start to get old they loose their accuracy ad start to lag behind the ECM which causes problems for the Corvette. If your O2 is older than 2 years of 50 k miles then replace it. The new O2 will help the Corvette get better performance and economy. A lot of Corvettes are out there with old oxygen sensors and running poorly because of it. Be very careful replacing the O2 as the new sensor can be damaged by many chemicals. When an engine runs poorly I start by checking the O2 to be sure it is still within the 24 months since I put it in.
If you replaced the sensor at the front of the plenum then you have replaced the CTS because that is where it was on my Corvette. I would check it's output to verify that it is accurate, those fail too.
On Amazon they sell a gas treatment called: BG 44K Fuel Injection System Cleaner and it REALLY works. I use a can every 2000 miles to keep the idle smooth as glass.
Here is the link to it.
Many years ago I bought the system to clean the fuel injectors on the car from 3M Corporation. The system connected to the fuel rail and then you pulled the fuel pump fuse to disable the fuel system. Then you start the engine running on the contents of a large spray can. After 6-8 minutes it would shut off and the engine was much smoother when put back on the gasoline. Too complicated and expensive. Try a BG product and it might really help your fuel system, I don't expect it would stop a leaky injector but it is possible it could clear the injector out and get the junk out of the the injector nozzle. BG makes several products and they are trusted by most every auto shop I know of. I would not suggest them if I had not had great experiences with it myself before.
I am getting the feeling that I might not describe the problem correctly.
My car will start hot but will fail to hold idle and will run high gasoline mixture if I keep it in 10 plus ram, it will not get to high RPM and will go without power, if I'll drive this way for a few minutes it will get better and the engine will eventually get back to work properly but it will take some time to recover.
I did not replace the sensor on the plenum. And what I am guessing is that the one on the attached picture is the one?
GM# 25036979 should be used as a reference for the MAT sensor. Your connector also seems to maybe be a problem. '85 to '90 I believe used what was considered a 'jumper' GM# 12029463. If the the 2 wire connector is complete then there's no issues.
Here's a eBay link that should be what that jumper appears like. I'm sending you a PM - I have a document that should help you ID your parts.
That is the the M*** Air Temperature sensor and it was mounted in a "bad" place on the L98's. To make more power you want the sensor to see "cooler" air temperatures. This usually means re-locating it to a better place where it is not exposed to so much heat. In an effort to find more power they are put somewhere they are not heated by the Hot aluminum Intake manifold and heat rising. WVZR1 sent you a link to the extension harness but they are easy enough to make if you are handy. Judging from what you are working on I would bet you are far more than just "Handy"!
When an engine has an injector leaking fuel under pressure, gasoline will build up and puddle inside the intake manifold. When you start the car "HOT" it might think it is flooded and you will have to play with the throttle to keep it running, it eventually burns of the excess fuel and then runs normally until shut down where the injector will start puddling the gasoline inside the intake manifold. Did you check the Fuel Pressure regulator to be sure it is not leaking?
Is the cold starting okay right now? Everything normal? How old is the oxygen sensor on your engine? Does your engine transition between Open Loop and Closed Loop without any issues? How does the engine run when it is cold? I wish we had some data to look at as that would make this job easier. The O2 doesn't start working until the Zirconium gets up to about 300* (C) and then it should send the oscillating signal to the ECM. Older O2's will sometimes take a bit longer to start generating the signal and this delays the transition to closed loop.
I realize they are more expensive where you are but I would seriously think about a new O2, especially if it is over 24 months old.
The next time you have a hard time when starting the engine HOT then try the Clear Flood mode and push the pedal most of the way while cranking and see if that helps at all.
Since your engine is apart judging from the picture have you removed the injectors? What did they look like at the tips where they inject from? My leaky injector had some markings on it to identify it as the leaker, The tip was darker in color than the other tips were.
Good Luck and keep in touch, we will get your Corvette running right yet!
That is the the M*** Air Temperature sensor and it was mounted in a "bad" place on the L98's. To make more power you want the sensor to see "cooler" air temperatures. This usually means re-locating it to a better place where it is not exposed to so much heat. In an effort to find more power they are put somewhere they are not heated by the Hot aluminum Intake manifold and heat rising. WVZR1 sent you a link to the extension harness but they are easy enough to make if you are handy. Judging from what you are working on I would bet you are far more than just "Handy"!
When an engine has an injector leaking fuel under pressure, gasoline will build up and puddle inside the intake manifold. When you start the car "HOT" it might think it is flooded and you will have to play with the throttle to keep it running, it eventually burns of the excess fuel and then runs normally until shut down where the injector will start puddling the gasoline inside the intake manifold. Did you check the Fuel Pressure regulator to be sure it is not leaking?
Is the cold starting okay right now? Everything normal? How old is the oxygen sensor on your engine? Does your engine transition between Open Loop and Closed Loop without any issues? How does the engine run when it is cold? I wish we had some data to look at as that would make this job easier. The O2 doesn't start working until the Zirconium gets up to about 300* (C) and then it should send the oscillating signal to the ECM. Older O2's will sometimes take a bit longer to start generating the signal and this delays the transition to closed loop.
I realize they are more expensive where you are but I would seriously think about a new O2, especially if it is over 24 months old.
The next time you have a hard time when starting the engine HOT then try the Clear Flood mode and push the pedal most of the way while cranking and see if that helps at all.
Since your engine is apart judging from the picture have you removed the injectors? What did they look like at the tips where they inject from? My leaky injector had some markings on it to identify it as the leaker, The tip was darker in color than the other tips were.
Good Luck and keep in touch, we will get your Corvette running right yet!
Hey,
So I did a head replacement on my engine 2 weeks ago (A lot of work) du to a little heating problem - apparently I had a bad head that was leaking pressure. I installed Edelbrock e-streets instead.
The issue was before the swap and still there.
I am going to get a new MAT sensor and put it in the air filter housing ...
I could not find any other temperature sender on the Plenum - Am I missing something.
Regarding your questions - I don't know when the O2 sensor was replaced and if ever ...
The car start cold perfect, RPM are kept OK and it drives fine.
I did put a new 165 degrees Thermostat and as far as I can tell the close loop and open loop transition is fine.
I will test the Fuel Pressure regulator on the weekend, I am trying to source a pressure gauge ...
When I removed the injectors and replaced the O rings on them I did not see any suspicious marks - but then again I don't have the expertise and did not think I should give more attention to them.
I will try to get O2 sensor from another car to test it ... worth a try as well.
Did you ever test/replace the injectors? I had similar issues and thought I had a leaky injector. I did an OHM test on the injectors before pulling everything apart and 3 of the 8 were bad! I was very surprised it even ran. Replaced the injectors with 8 rebuilt bosch 3 injectors from fuelinjectorconnection.com and she purrs now. Before you start replacing everything else test those injectors.
On the 1988 C4 they had a Cold Start Injector that activates anytime the engine is under 31* (C) or some weird number. The cold start injector has a Thermo Switch that activates the Cold Start Injector (also know as the 9th injector). I will find my Corvette Books and look up the locations on the 1986. On the later Corvettes they had a Three wire sensor that is the Coolant Temperature Sensor on the front of the plenum on passengers side (I will Check).
The one you circled I believe is the M*** Air Temperature sensor. The ECM software uses both sensors but the CTS is the biggie as it is used all the time with the Oxygen Sensor and the MAF of MAP (if you have one of those).
I think that your engine has a injector that is leaking but it ISentirely possible to have similar symptoms from a over-active Cold Start Injector. Take a look before we go out and buy new injectors. I have seen situations where the CSI was activating when an engine is warm. With a leaky injectoror a 9th injector activating you will have to hold the gas pedal and "play" with it to keep the engine running until the engine gets warm or burns out the extra fuel. On my 1988 C4 the CSI system is located on drivers side towards rear of plenum and the injector is close as well. There is a electrical connection that activates the CSI and I would try and disconnect that before going to far. It is a simple Injector harness connection to disable it. It Might generate a code or two but you are testing it. By the way, that is just another Electronic Fuel Injector... and they are not cheap so I had mine rebuilt and I bought a spare.
Try all this and let us know what happens! The Oxygen sensor is just one of those things like gasoline and spark plugs. That sensor can make a big difference if your old one is not working well. It spends it's whole life calculating the air fuel ratio for the ECM. Improves economy and even performance all for $25 U.S..
Engine problems when starting hot
