Hey guys,

newbie here. I’m slowly running out of ideas so I thought I would ask here. Any advice would be greatly appreciated.

First, some details about the car:
- L98, 116k miles
- new fuel tank + sending unit
- new fuel filter
- new evap canister+ valve
- new fuel pressure regulator
- new EGR valve

So it was running pretty great after the fuel tank replacement and everything else mentioned above but it was starting a little rough on hot. It would start instantly on cold so after my friend’s suggestion I replaced the IAC valve, cleaned the TB, did the system reset, adjusted TPS and it kinda idles rough now. And definitely starts even worse on hot. What is more it does not want to accelerate while driving, it’s choking and looses power when I press the throttle more than 25%.

Any leads would be very helpful!

Cheers!

 

With a dmm check the tps resistance thru it's entire sweep. Open the throttle slowly but steadily, the dmm should reflect that steady sweep. If resistance jumps abruptly anywhere, the problem is in the tps. It's a quick easy place to start diagnosis

 

thanks for the reply! I did check that, it is linear. Did not see any problems there :/

 

How strong is Vacumm on some available ports, is there a rattle from under or a smelly Exhaust. Because some of the things could indicate a bad or failing Catalytic Converter

 

Thanks for your suggestion!

Not sure how to precisely check the vacuum but what I did notice is that there is a good air pressure in the tank. Catalic converters are old for sure and I know that the interior gets really hot after a long drive. I know these cars do that but it looks like cats might be getting too hot. Any way to check that?

Thoughts on that?

 

this is just a quick guide to help you along Google / Bing & You Tube have many how too's

. https://www.magnaflow.com/blogs/magn...erter-symptoms

. When you say it looses power is it trying, or is it stumbling and breaking up. I had a car come in with a complaint car lacks power, it would get to about 2500 RPM+ Nose over and Stumble and break up by 3000 RPM.

. It turns out it was the coil

 

It is possible that the Catalytic converters are getting blocked or are plugged. Get an infrared thermometer and see how hot they are. The temperatures should be close and consistent to each other. Another test is to remove the O2 sensor and see how much air escapes out of the hole. It should be ~5 psi not a blasting air force like a plugged exhaust.

I would unplug the M*** Air Flow Sensor and see "how" the engine runs. If it is a bad sensor your Corvette should run better without the MAF attached. Sometimes the two relays that operate the MAF also can act up. If and when you replace a MAF always replace the two relays. Often a good cleaning with MAF Sensor Spray Cleaner will help clean up an abused sensor. The wires in the sensor are very tiny 1/10th of a mm and are very fragile, use the spray cleaner to be safe!

How old is the Oxygen sensor? If it is over 24 months or 50k miles since it was last replaced I would spend the $25 and buy a new one and install it. This sensor can wear out and does wear out, they have lasted longer than 24 months but GM felt by 24 months they were worn out enough to "justify" replacement.

A good working coil always helps but getting the power to the spark plugs has turned out to be a bit more difficult. Spark Plug Wires can fail and limit the strength of the spark. If in question I will measure the size of the spark the system produces using a tool that goes in-line with the spark plug wire to the ground. It has variable gap size for testing the spark strength. Another valuable test would be start the engine when it is dark and watch for sparks on or near the engine while running.

Have you upgraded the original fuel injectors yet? The originals were very easily damaged by the ethanol in our gasoline. Mine were less mileage but leaking already so you might want to get a set and install them. I have a set of Bosch injectors from South Bay Injectors and they work just fine. The extra fuel from dripping injectors when the engine HOT is often what makes it hard to start. You have replaced so many parts already these should have been on your list as they are important to a good running L98.

There is a third sensor that is also very important to you L98 running properly. The Coolant Temperature Sensor is one of the more important sensors as it's signal has a huge effect on how well the engine runs. It should be checked every so often as they wander when they fail. If the sensor tells your ECM that the engine is 190* when you start it up it will give your engine very little extra fuel to start and this makes cold starting more difficult.
Had the sensor failed thinking it was 40 * (F) it would add extra fuel to help a cold engine start, once warm the engine would start running rich as the ECM will give it more trying to warm up the engine. If the engine were warm already then it would be very hard to start. The CTS signal is used in many many decision by the ECM and there for is one of my top three. O2, MAF and CTS are the big Three required to drive smoothly in Closed Loop.

To save yourself a lot of time and money find a OBD1 scanner and look at the information "live", problems are more easily found this way. The Factory Service Manual is something you probably have by now, If Not they even sell the DVD version on RockAuto.

 

Thank You ctmccloskey! I appreciate your detailed response. I have been working on the car for a while and although I haven't figured it all out yet, the car is still choking and loosing power while driving, I have isolated a few problems so here's an update:

- Replaced all the vacuum hoses and the HVAC valve (most of the vacuum lines were original and started rotting out) Not sure if that eliminates the vacuum leak issue yet?
- Replaced TB and plenum gaskets, cleaned the TB and the Air Sensor in the plenum, everything torqued back to factory specs
- Replaced a previously purchased IAC valve (GM) which started all this trouble in the first place now the car idles better after it warms up so it definitely added to the list of issues
- Checked the the fuel sending unit just in case, all good, everything in place, no leaks, fuel pressure is good
- EVAP system checked, it keeps the air pressure and there's definitely a good amount of air going back to the tank (new canister, valve and hoses)
- Replaced the O2 sensor
- Checked and cleaned MAF sensor, the engine light came one when I started the car with MAF unplugged, it kinda revs a little quicker but still far from normal so I'm guessing it's not that? Cleared the codes, plugged it back.
- Checked the TPS again, works linear, set to 0.54V

At this point I might as well dive into things you suggested but here's what baffles me. Everything seemed fine before I touched the IAC valve and I also cleaned the engine bay after that. No high water pressure but maybe something is shorting somewhere now?
All this feels like electronic/ sensor-is to me.

Considering Tesla swap now...

 

I may have missed it, but did you check your fuel pressure at the rail? I know that you just replaced the sending unit. But, could it be something as simple as a failed fuel pump right out of the box?

 

Hello again NightRyder1983,

This may not be part of the problem but did you change the IAC? IF and when you did this did you verify that you got the right shape "Pintle" (the pintle is the tip of the IAC that moves). I went to a NAPA store and they gave me the wrong shaped pintle for my IAC. If you get the wrong IAC it will potentially not fit the throttle body air passageways. It turns out that the 1988 year was when they transitioned to the newer style Pintle and that would clearly mess up your fuel injection. Can you compare the old one to the new one?

Have you measured the spark yet? I would consider checking the spark plugs with a tool that lets you adjust the gap to see how much the spark will jump. It attaches to the spark plug boot on the wire and the other end needs a good ground. You can then adjust the gap to what ever size you want for testing. I would do something like this to verify that you have a HOT spark. BE Careful with the ignition system as a direct hit from it might put you on the ground. Don't play with HEI systems. This test would also let us know if the coil is still working properly. You might run the engine at night and look for any sparks in your engine compartment as today's spark plug wires don't seem to be as rugged as the older ones.

How long did the problem take to start happening after you replaced the fuel tank? The problem you describe could easily be a Catalytic getting plugged up. Measure the temp of the Catalytic with an Infra-Red Thermometer

Please test your Coolant Temperature Sensor and maybe even the Intake Air Temperature as well. The CTS is the more important of the two but they both have an affect in the air-fuel ratio. The fact that the Engine performance changes based on temperature make me check the sensors indicating temperature to the ECM.

Is there anybody with a OBD1 Scanner that could show you some real time data? That would be really helpful when the car starts to act up. This is when I would have my laptop running Tuner Pro RT in the passengers seat all wired up to catch the action "live".

By the way, I am an Engineer trained in ISO 9000 manufacturing process'. The more parts you have the more likely it will break. A Tesla car has thousands of 18650 (a bit bigger than a double A battery) Batteries in them, now they are using a slightly bigger version but still thousands of them.

I don't think Nicola Tesla (Mr. Alternating Current) would like his name on some (Direct Current) battery powered vehicle. Elon should have honored it and called it an "Edison" and not a "Tesla". Tesla has been my Hero for decades and I have read a lot about his life and my former employer (Westinghouse) and their interactions to beat Edison with his lame idea of a DC generating station every Mile just for lighting our homes. Tesla should have been given more credit for his discoveries and honored for all eternity.

 

Hey Guys,

Big thank to all your suggestions. I dived in to the fuel aspect and something was telling me that the fuel pulsator failed and it actually did. This part is useless to be honest and I don't even know why I installed it back with the new fuel sending unit. Anyways, checked the fuel pressure on the rail, it is hitting 40psi but it is dropping quite quickly if I don't start the engine. Stays there with the engine running. So is that a sign of the injectors leaking a bit? The car runs great, starts right away cold or hot. Throttle response is great. Vacuum is great. Replaced IAC made it idle nicely. Tesla swap was a joke, not to offend anyone of course.

Cheers!

 

I have a spare pulsator sitting on the shelf new in the box. When I did the new pump on mine, I just replaced it with some good submersible fuel line. I couldn't see the point of putting it back in either.