C4 engine has no power

I took my 1990 C4 out of winter storage last week. I drove it about 50 mile the first day with no problems. The next day it will only idle. When I press the accelerator pedal, the engine loses power and
stalls unless I release the pedal where it settles into an idle again. Is this a fuel delivery problem? I put clean fuel in it the day before. I'm thinking a clogged fuel filter, a bad fuel pump, or a defective fuel pressure regulator. Or maybe it's none of the above. Any opinions?
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SteveC wrote:

All tempting clues. Add in the Throttle position sensor on the throttle body (easy check with a multimeter).
If the fuel filter hasn't been changed in a few years, I'd go for that first. If it's a convertible or a coupe with a frame brace check the filter access. You may have to jack the car in an area where you can clear the frame brace from the filter.
Do not (!!) run any injector cleaner through the fuel system. Injectors on 88-90 are easily damaged by injector cleaner or Ethanol content over 10%. The windings in these injectors are cooled by fuel and solvent/alcohol eats the insulation off the wire, leading to shorted injectors and an excessively lean condition.
Keep us posted on what you find.
-- pj
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Today I checked the signal from the throttle position sensor. It reads from 0.5 volts to 5.0 volts depending on it's position, so it appears to be working and in spec. There are no 'service required' codes being set, and the code 12 being present shows that the ECM is working. I put a pressure gauge on the fuel pressure port and it showed a pressure of 35-40. When I stepped on the gas pedal, it climbed to about 50, and of course the engine stalls when I step on the gas. It appears that the fuel pump is working. I haven't been able to replaced the fuel filter yet, but that's on my list of thing to do next. If there was damage to the fuel injectors, wouldn't it be unlikey that all 8 failed at the same time? Since the engine gets enough fuel to maintain an idle, isn't that an indication that the injectors are working? I'm running out of ideas.
sc
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wrote:

stuck iac valve? there's a test sequence for it in the manual but i disremember it
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wrote:

I had a lesser case of teh same with My LT-1 that turned out to be a bad "Ignition module" or the solid state driver for teh ignition coil. Would step on gas and at pretty hard throttle engine would die.
Not sure if the 90 (L98??) has the same, but there's another one..., coil or coil wire, as I remember , the voltage goes up as throttle does (to a point) and may be breaking down under the higher voltage.
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wrote:

i think it could. it has to be open/closed the right amount or too much air is let in, causing a stall because the fuel/air ratio would be way off.
i had a problem with my 94, that it would cause bucking and hard to keep running under medium to high throttle. it turned out the tube that runs from the top of the engine under the passenger fuel rail cover to right behind the aic into the throttle body was cracked. under high heat, the crack would open allowing too much air in, causing a stumble. when it cooled off, the problem went away.
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charlie wrote:

IAC is a possibility but I'd think that idle rpm wouldn't be repeatable between cold and hot condition. IAC usually throws a code if it isn't cycling correctly at start.
Unlike the C5, two things on my '89 that don't throw codes are the cat and the injectors. The '90 is probably the same.
Just for giggles and grins, make sure that the air filter hasn't disintegrated and isn't blocking the MAF screen or throttle. IIRC the '90 may not have a MAF so, I'm guessing about what's behind the filter.
Since this was stored for winter and driven once, a cat failure or slug of sediment into the fuel filter would be tops on my radar.
Check the cat for clogging or making a rotten-egg odor. The shop manual has an exhaust back-pressure test (measuring at the air-pump ports or at the oxygen sensor.)
Next, I'd check the injector resistances since that will be easier than changing a fuel filter.
Injectors: following pertains to the '89 but might also be true for the '90. First, make sure there's no corrosion at the INJ1 and INJ2 fuses. (On the '89, the service-engine-soon light works off the INJ1 fuse. 1-3-5-7 are on that fuse, the even injectors are on INJ2.) (Fuse provides the +12v and the ECM provides a pulsating ground.)
The '89 alternates between banks, firing all the odd-injectors, then all the even-injectors. The '90 might do the same. Individual injector firing for the L98 came later. Possibly in '91 or with the Optispark system in '92.
On the '89, if one injector has low resistance, it weakens the "firing pulse" for all injectors in that bank. Result is that all four cylinders get a lean mixture, not just the cylinder with the bad or shorted injector. The result seemed worse as the throttle was opened and as the bad injector heated up, the engine leaned out and quit without throwing a code. (sound familiar?)
Check the resistance of the injectors, first when cold then let it idle until it's really warmed up and quickly check them again before the fuel inside them cools the windings.
I couldn't find the correct resistance in the shop manual. IIRC it's either 13 or 15 ohms. <http://www.fiveomotorsport.com/FMC_Compete-Set.htm shows it as 12-14 but that's a 'stang site talking about 'vette engines.
In my case, an injector in one bank was 10 ohms cold and 6 hot. In the other bank there was one with 8 ohms cold and 4 hot. That was enough to give a smooth idle but little more than a couple of miles an hour in low gear, then it would die.
If you have Multec injectors, individual replacements must be p/n matched to the others in the engine ($$$). Check with Corvette Forum for substitute suggestions or email me.
If the injectors look good (both cold and hot) replace the fuel filter.
You might also get some good tips from the gents in the C4 Scan and Tune section of the Corvette Forum <http://forums.corvetteforum.com/zeroforum?id >
Anything beyond simple resistance checks of the injectors begs for a 1990 shop manual. If you don't have it order soon, before HelmInc gets sucked up in the GM BK mess. It has very clear diagnostics for each component and neat troubleshooting charts for the whole emission control group.
HTH!
-- pj
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WAIT!! Before you go through all of that. Put a vacuum gauge on it and see if it pulls a vacuum when you open the throtlle. If not check the Cats they may be plugged up. Plugged cat = no flow. If exhaust can't escape intake air can't enter. Intake can't enter engine starves.
Fred
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The Mass Air Flow sensor determines the amount of air entering and the computer adjusts the fuel accordingly. If that was bad the engine would lean out as the throttle was opened. Not all cars have a MAF and if yours does not, then that is not the trouble. I am not sure how to test it other than to substitute a good one. Manual may indicate how to test.
Vito
wrote:

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Been a while since I did diagnostics on an car of this era, but from your description you have an severe air/fuel mixture problem. With a problem this bad just about everything on the intake side will throw a code if there are problems there. If the MAF was bad, the computer would detect this and go into closed loop and throw a code with these symptoms. It's very easy for the computer to detect a MAF failure becaues when the TPS opens, the airflow through the MAF should increase. If increased airflow is not detected, then the computer knows the MAF is not working and will go into closed loop. A flaky MAF that does not throw a code (one that is working but reporting incorrect airflow) will usually cause poor gas mileage and reduced power, not the problem you are having. You can also verify that the MAF is NOT the problem by disconnecting the wired connection on the MAF. This will throw a code after a few minutes, and your "check engine" light will come on. The system will then be in closed loop and if the problem goes away (or gets a LOT better) then you do have a bad MAF. After doing this test, disconnect the battery ground for 5 minutes to clear the code and reconnect the MAF connection. I seriously doubt you have a bad MAF because it wouldn't cause a problem this bad without throwing a code.
I would pull apart all the pieces between the MAF and intake, could be a dead mouse in there blocking (or disturbing) airflow or a vacuum leak. With the severity of the problem, you should be able to hear a vacuum leak (a whistling noise) with the hood up and manually opening the throttle. There are one or more vacuum connections on the intake manifold, so you need to check those connections and associated hoses, but again, with a problem this bad you should be able to hear a leak.
You could also have an blocked exhaust, not sure how to easily find this other than taking it apart or using a camera system used by plumbers to look inside sewage pipes. I doubt this is the problem because a clogged exhaust would also throw codes on the intake side because air can't come in if it can't go out.
I also doubt you have a fuel filter problem because your fuel pressure looks good.
I'm would have to go with the one of the other posters suggestion that you have one or more bad fuel injectors as this would not throw a code and would present the symptoms you are having. If you do replace the injectors, replace all of them because if one is bad, odds are that another will fail soon. I think you need to pull the intake manifold to replace the injectors which is a lot of work and another good reason to replace all of them.
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After ruling out several possibilities, I narrowed it down to the injectors. I ordered a set of new injectors from JEGS and they arrived by FedX yesterday. Today I installed them and put the engine back together. It worked. The engine has never run better and has never had more power. What a difference. Thanx to all who gave me tips and advice.
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After ruling out several possibilities, I narrowed it down to the injectors. I ordered a set of new injectors from JEGS and they arrived by FedX yesterday. Today I installed them and put the engine back together. Took me about 8 hours but I'm no mechanic. It worked. The engine has never run better and has never had more power. What a difference. Thanx to all who gave me tips and advice.
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SteveC wrote:

Did you store it where something (rodent) could have moved into the exhaust or intake areas?
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Yeah, it was stored in an old chicken barn that is full of cars and boats now. No more chickens, but I did see a dying mouse the day I left the car there. I did check the intake for rodents, but have not checked the exhaust. Thanx for the idea. The air filter is in good shape. I'll be working on the car again tomorrow. I plan to check the resistance of the injectors, and also see if there is a spark when I rev up. I do have the service manual, which has been a lot of help. I can see that one injector looks like it was replaced. It's a different color. I have owned the car for only two years. The first summer I replace the EGR valve and then all was well. It has always idled with an oscillating surge. Rev up, rev down if you know what I mean. That must be a symptom of something. I'm still trying to disconnect the fuel filter, but I'm having trouble getting a good hold on it with wrenches as I lie on my back under the car eating road dirt droppings.
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Today, 4/15, I was able to R&R the fuel filter. It didn't help. I also measured the resistance on the injector coils. It's ugly. In ohms they read the following: 10.6, 17.0, 12.2, 1.5, 5.3, 16.6, 7.5, 10.0. I don't know what the spec is, but this doesn't look good. Could this be causing the no-power problem, or is this just another issue I need to work on? BTW, this model year does not have a mass airflow sensor. It seems strange that this vehicle worked fine one day and not the next.
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SteveC wrote:

Looks like both banks are in bad shape. Mine went from good to dead in about 20 miles of driving.
Check out the dialog between Rich Jensen (no relationship) and Sam Lam in the Corvette Forum.
-- pj
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I got the vett running today, but I dont know how I did it. The service manual described a method to test the control module to see if it will set a code by disconnecting the throttle position sensor and run the motor for a short while. I disconnected the TPS and let the motor idle. The check engine light did not come on. Next I borrowed a set of little lights for connecting to the injector wires to see if they are getting a signal. I tested each injector one by one by connecting the injector wire to the light, turning on the engine and watching for the light to flicker. They all worked. After I was done with that I cranked the engine over once more and let it idle. I stepped on the gas, and guess what. It revved up nicely and didnt stall. The engine started to run! Why I dont know. I drove about ten miles and the engine felt like it was missing a cylinder. The check engine light came on. It had set three codes. Codes 22, 44 and 45. Code 22 is for the throttle position sensor, the one I was trying to set. The other two are for the left oxygen sensor. More work to do.
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