When I start my 91, the rpms drop until it almost stalls, before it jumps up and falls again. It's sort of a rythmic up-and-down, where it almost stalls, catches itself, and goes back.
Fuel pressure tester indicates 42PSI at the rail, which immediately after the fuel pump shuts off drops down to 38 or so. It'll drop of slowly from 38, but fast enough that I can see the needle moving.
This is happening after the engine started to stumble at low acceleration, causing bucking and roughness. Under high load or throttle, the stumbling would smooth out. I can't keep it running long now, but while it was idling I could smell raw fuel from the engine bay, and there was excessive backfiring leading me to believe it was running super rich.

Any ideas are appreciated. Thanks!

 

The Bucking is likely caused by the MAP sensor on your Corvette. It is not expensive and you should be able to change it yourself.

Have you checked the Throttle Position Sensor and verified the IAC is working?

How about your Coolant Temperature Sensor? Is it accurate?

Backfiring can be caused by lean conditions as well.

 

Any codes/Check engine light?

 

@sheriffjim how many miles are on the car? Your fuel pressure loss combined with rich fuel smell indicates you may have an injector leaking and pouring fuel in to the combustion chamber. Since you have a TPI car, pulling the injectors is a bit more involved. Have you pulled the spark plugs? if so, how did they look? I think before you replace the MAP sensor, a check of the spark plugs is a good starting point. @ctmccloskey is right, the MAP is easy and not expensive I would not make it the first item to check and replace. Keep us posted!

 

I already replaced the MAP sensor because it had a code. Did not fix the issue, of course The spark plugs have lots of black buildup on them they are definitely bad. I'll order a new set. I have just replaced my O2 sensor, manifold temperature sensor, and coolant temperature sensor because of codes stored in the ECM. None of these changed the issue, but the SES light is gone.
The car has 220k miles . I'm saving some money to rebuild the engine, but would really like to drive the car in the meantime.

 

Hey Jim. The black build up on the spark plugs combined with the rich fuel smell pretty much points to a leaky injector and possibly more than one. Once you get the new spark plugs installed, do the check for any arcing as that should be an easy fix if there is one. At over 200K miles you got all the service out of the original injectors. You can ohm check them before you pull them. I don't know which injector people are recommending for the TPI cars these days but set aside a day for the injector swap as the process to replace to tear down and reassemble the multi-piece TPI intake takes time, patience, and attention to detail. In addition to the injectors you will want to get a full TPI gasket set and make sure you have the right line wrenches for the fuel lines. Even with the high miles, I don't see any reason why you can't enjoy your car once you overcome this issue. Keep us posted on your progress and good luck!

 

Ive already ohm tested the injectors and all are fine. I was suspicious of bad injectors however, as I’ve heard about their poor quality. (Also it appears a previous owner replaced SOME injectors but not all, indicating that some had already failed.) I’ll get to work and update as I make progress.

thanks for the help!

 

An injector can ohm correctly and still leak. Dan

 

On my 1988 C4 I had a leaky one that passed the Ohm measurement test but leaked like a sieve when I tried it off the engine. The tip was black and looked different than the other injectors. I put 50 psi of air in place of the Fuel at the inlet and then attach power to each injector to see if it is leaking. I put the tips in a bowl of water and watch for any leakage since I have a hearing loss and cannot "hear" them leaking. When bubbles appear out of the tip under water then you know it is a leaking injector. The Injector re builders use a non-flammable liquid with similar properties to gasoline and measure the flow rate and watch the spray patterns amongst other tests. Getting your injectors tested costs a bunch but provides good evidence of a problem. It is cheaper to just buy a set of injectors and put them in. It is not a hard job to do, you just have to keep the area clean around the injectors and their O-rings. The air pressure test is an easy to perform test at home.

South Bay Injectors carries the matched set of eight 22 lb Bosch D3 injectors for $175 and about $450 for a set of brand new injectors. I have always used the rebuilt injectors and they worked great. There are plenty of companies that sell quality Fuel Injectors so try to avoid Ebay and Amazon on these items. Buy them from someone you can trust and will help you in case of any issues.

The reading from your Intake Air Temperature sensor is important to your engine running cleanly. It is located at the rear of the plenum and can get hot from the proximity of the engine's heat. The hotter it thinks it is it will lean out the mixture. A relocated IAT can help your engine see cooler air coming into the engine can make more power since it is not being pre-heated by the engine block's heat.

Good Luck Sheriffjim, we are trying to find any way to help you so your issues will go away and leave a "Fun to Drive" Corvette!

 

Yep, I bought some re manufactured injectors. They're ready to go in the car but it might be a few days before I can put them in. The IAT sensor was relocated by a previous owner, I got an SES code for it and replaced it recently (still in the relocated area).

I replaced the spark plugs today and they came out very wet with fuel. Whenever I get the time I'll replaced the injectors, as well as the fuel filter and any intake gaskets I have to take apart while I'm doing the job. I'll update after I have the new injectors in to let y'all know if it works. Thanks for all the help!

I noticed a hose on the passenger side valve cover that came disconnected somehow. It looks like a press-fit type of thing, and even after I put it back it fits pretty loose. It looks like it goes to the throttle body. Any ideas as to what this is?

 

It's the source for the "make-up" air for the PCV system - the driver's side valve cover has the actual PCV and a hose to the intake manifold.. When the PCV system is drawing a vacuum succesfully, this line on the passenger side will pull in filtered air from the main air filter. In the event that blowby exceeds PCV capacity, it will vent excess pressure (in theory) up to the throttle body where it can be re-ingested by the engine. Note that the "somehow" it became disconnected by could be excessive crankcase pressure from blowby. I think maybe back-firing is a possibility there as well?

 

Ahhh I see. Thanks a lot, I'll figure out how to fix that. Backfiring would be my guess for what caused that. I'm not really sure why it would have excessive pressure. I'll look into it. Thanks!

 

Ok new injectors are in. The problem is not resolved. I no longer have the fuel pressure tester (was renting it), but the issue is going on the same as before. I'm going to try and attach a video of the issue but I'm not really sure how attachments work. I'm a little bit at a loss as to why this is happening.
There was a strong smell of gasoline in the plenum and runners when I removed them, and the car still smells like gas when started.

 

I think that this point - if you have the ability - it might make sense to hook up a cable and log some data while it's doing this. I wonder: Could you have a bad throttle position sensor? Is the ECM perhaps trying to respond to what it believes is a movement in the throttle cable? Data log would show what's going on with the various sensor readings. Given the money you've put into solving this so far, a Moates ALDL cable would be a comparatively small investment.

Just so you know this isn't totally out of left field, here's a clip from the FSM for a 1990:

 

That could be possible. It's not just having idle issues, it also refuses to rev. It'll backfire and won't rev up if I give it throttle. Could this also be a TPS symptom? I'll look into the ALDL cable, it would be useful to know what the ECM is thinking.

I'm also not exactly sure what cable I need or what I need to use it. Any advice with this?

 

I had a c4 where the fuel pressure regulator would send gas into the engine via the vacuum line. Something worth checking because it takes about 30 seconds

 

There is gas in the FPR vacuum hose but it's a brand new FPR. Not sure if the gas got there from the FPR or if it leaked into the hose from the plenum. I could try and replace it again but that was fairly expensive and I would prefer to avoid that if it's possible.

I should mention that I just replaced the diaphragm. I don't really know how it works but is it possible some other component of the FPR is broken?

 

There should only be air on that side of the unit. If there is gas going into the vac line you found the problem. That dumps fuel into ALL cylinders which lines up with your fouled plugs (vs just having 1 or 2 fouled).

Assuming this is it you should change the whole unit.

To test if you can VERY CAREFULLY plug the vac line (both sides) securely it will run without it just fine for testing. The careful part is serious because if the gas shoots out the exhaust is where it will land and that is a fire waiting to happen

 

Ok, I'll look at getting that fixed. Thanks for the help!

 

I bought the combo from Moates that included their "ALDU1" and "CABL1" products. The ALDU1 is basically a USB-to-serial converter that handles the communications and the CABL1 is just to make the physical interface to the ALDL port. I use TunerPro (which is free) for actual datalogging.

http://www.moates.net/aldu1-and-cabl....html?cPath=97