Hello i am new to this forum any information would be apprecaited,i have been trying to resolve this problem for 3 months , my 90 l98 has developed a sluggish idle and acceleration problem, cranking the engine has been taking an increasing amount of time start, had a bogging incident where it almost died out.. replaced injectors with standard fj 47s new, fuel pump and filter, (regulator shows no symptoms of failer). Vacuum connections are good, new plugs-wires cap and rotor. Compression shows 150 accross the board. thows no codes.. just flashes 12 when in diagnostic mode, from what i understand this is normal.. has anyone had this issue and or a remedy? Thank you 😁

 

Did you test your fuel pressure? Or just throw parts at it?

have you tried resetting the computer and letting it relearn the idle and let closed loop do it’s thing?

have you checked your throttle position sensor? They can get dead spots and cause all sorts of issues.

have you checked your base timing?

what led you to assume it was fuel related?

compression sounds fine. Have you thought of doing a leak down test?

 

Yes sir the fuel pressure was one of the first places i saw trouble, it showed 40 psi but droped off quickly, combined with a faint sent of fuel on the oil dipstick i concluded one or more leaky injectors ( cyl 5 and 6 showed 9-13 ohms as opposed to 17 ohms from the rest of them). Still no fix so jumped on fuel pump and filter still same issue, rechecked wire connections at plugs and coil.. the strange thing is that it runs clean for the first 10 seconds or so then begins to bogg.. i rebuilt this engine last year and refused the original distributor , and the valves never got done.. I'm beginning to think a valve could be the issue...?

 

Hello Elivett!

Welcome to the Corvette Forum!

When you say it runs clean for the first 10 seconds or so then begins to bog are you suggesting it runs fine for the first 10 seconds and then bogs?

There are three important sensors required for your Corvette to transition to Closed loop. One is the Oxygen Sensor. The O2 is an item that has a limited useful life. The folks at GM wanted us to replace the unheated O2 every 24 months or 50k miles. We have seen as the O2's go bad they tend to send a rich mixture by default to avoid hurting the engine. If your O2 is over 24 months old, replace it as it cause problems as the data gets slower and the signal less strong over time.

The second device of the three is the MAF or MAP sensor depending on which your Corvette came with. The MAF/MAP basically measure the volume of air going into the combustion process. If there is any air leaking between your MAF and the throttle body it will cause drive-ability issues. The MAF on my 1988 C4 has two relays to operate it. One powers the sensor and the second relay controls the "Burn Off Function" which is done to clean the wires used to sense the air flow. If the MAF is dirty you would have bogging potentially, you can clean it with a special MAF cleaner spray that should not hurt the tiny wires used in the MAF. The wires are ~1/10th of a strand of hair and they are easily broken by aggressive cleaning.

The next part that is critical to your Corvette running properly is the Coolant Temperature Sensor. The CTS is very important to the fueling of your engine. If the CTS is erroneously sending a signal of 30* when in fact it is really 80* the ECM will try to add fuel to start the "Cold" engine. This will make the car harder to start when Hot as it will be like having your Choke stuck "On" all the time. There is a chart in the Factory Service Manuals that shows the resistance value measured and what temperature it corresponds to.

When replacing the injectors on the L98 it is important that they are the right size for a stock L98. I think it is either a 22 or 24 pound injectors I used in my 1988 C4. I bought mine from South Bay Injector and they shipped fast just down the east coast. Find a quality supplier to buy injectors from, I am hesitant to suggest EBAY as I don't trust their quality.

Your compression numbers are a bit low but as long as they are all within 10% from the highest number to the lowest number. Did you do the compression test did you secure the throttle wide open and attach a battery charger to the battery? The battery helps the consistency of the numbers by all being charged up at the beginning of the test. You might also try putting a little oil in the cylinder and get the wet reading. Having both wet and dry is helpful. I doubt it is a valve causing this, one way to find out is to attach a vacuum gauge and watching it for a minute or two.

Flashing a Code 12 is showing that everything is "normal". Nothing to worry about.
I suspect your issues might be coming from more than one place. I would really go over the MAF to ensure it is working properly. Since there are no codes I will assume it might be okay. Bogging is a effect that can be brought on by a Bad MAF or even a failing/failed Oxygen sensor.

Do you have access to a scanner that will work on the OBD1 systems? I have an old Snap On MT2500 and it is really helpful when you have problems that are hard to diagnose.

Be sure to let us know what you find when working on your C4, this way it helps all of us here at the Forum!

 

Also my coolant temp sensor is about 8 months old however.. the wire insulation is melted from the exhaust manifold. I overlooked this for a long time due to my guage reading it the same as it always has.. also i have some live data info on a video...i also have fsm on hand, is there anything specific i should look for other than o2 and coolant temp? Thank you for your time, this problem has me stumped

 

Thank you for these points of possibilities, it does run healty for the first 10 seconds or so somethimes a bit longer, then will start spuddering.. i put a new 02 sensor today but problem persists. Also found that i did the compression test wrong the first time... had someone help me with wide open throttle with plugs pulled.. we found that cyl 2,3,4 are about 40 psi low on first crank but quicky after they all acheive at least 160 psi, another words they are a couple cranks slow to full compression. I do want to check temp sensor next it seems to run worse the hotter the air gets, fan is struggling to keep the engine from overheating , granted we are in a heat wave this month

 

The coolant temperature sensor wires are burned by your exhaust? The CTS has three wires going to it and all three need to be protected from getting grounded. Be sure that they are all three insulated and not grounding anywhere.

The compression being 40 psi on the first rotation is not what you are looking for on a Compression test. We need consistency so I attach a battery charger to the car and wire the throttle body open before starting the compression test. The final number is the most important. If they are all within 10% of 160 psi you are okay, it is when they are outside of the 10% that we start to get concerned.

An engine that runs hot quickly might have a clogged or plugged Catalytic converter. Remove the O2 and see just how much exhaust blows out the O2 hole, it should be a little like ~5psi leaking out there. IF most of the air wants to go out this way then you have a plugged catalytic. Another way is to check your catalytic converters when the engine is running.

Timing being off can make a engine run hotter. Bad gasoline can make it run Hotter. A faulty EGR valve make the cylinder combustion temperatures go up and pinging will start. Excessive anti-freeze in your Corvette overheat as will a bad radiator cap that doesn't hold pressure any more. Radiator caps and thermostats are often only replaced when they fail. I like to replace the coolant, thermostat and Radiator cap every ten years just to be sure it all works the way it should.

Try measuring the temperatures going into your radiator and then measure the output and see how different they are. Please let us know the answers to allow us to help. Hold old is the thermostat? How old are the hoses on the engine? Is the spring inside the return hose like it should be?

It is highly unlikely that one of the valves is causing the problem. The CTS is still a suspect in my book from what you have told us. A plugged catalytic will cause you all sorts of trouble so verify with the O2 removed.

What bothers me about your issue is that it occurs Ten seconds after startup. At this point your engine should be using stored tables of data running in "Open Loop". Do you ever see the engine going into the Closed Loop? Can you describe what happens when it tries to get to "Closed Loop". From the way you described it I would guess it never gets too the Closed Loop or does it? The only sensor that has to heat up is the O2 sensor and when it gets warm the system goes to closed loop. If the Corvette is not entering closed loop it could be because of a faulty sensor like the MAF or CTS or even a bad O2. If your Corvette has a MAP sensor then try replacing it. Or is your Corvette transitioning into Closed Loop and just running too hot?

One way or another someone here on this great Forum will know the symptoms and jump in, if not I will continue to try and help you. We will prevail!!!!

 

Thank you i appreciate the in depth analysis on these topics , yes perhaps i am overthinking the valve/compression scenario.. the hoses/water pump/t stat and water neck are fresh from the engine rebuild, the wire that is melted it the one sending the guage and shouldn't send signal to the ecm( i might be wrong though)..I did follow your advise as it made sense as far as ecm commands by replacing the coolant temp sensor underneath the throttle body, i did a new map sensor and also a fresh 02 sensor ( i work in autoparts so i get break on prices) however something still seems to throw the engine into studdering after it warms up.. the CAT is original and i haven't considered that a possible suspect 🤔. While replacing the O2 i ran it for a couple of seconds unplugged , 8 heard a pretty nasty growl come out.. Maybe i should try to unbolt the CAT and see how it runs?( i know it will sound terrible but at least then i can see if it's hindering the engine aspiration.. Also.. how can i tell if it enters closed loop? It's almost like it craps out WHEN it goes into closed loop from.