RexS

I have a largely stock 1990 L98 Coupe that I just bought. It had been sitting for a while and the PO had it towed to a garage where the mechanic cleaned out the fuel tank, replaced the fuel pump, sock and filter. The car starts well and idles evenly.

I live about 2 miles from a freeway. I can drive there, hop on the freeway (she pulls hard!) and she will run at high speed indefinitely. On the other hand, if I stay in town and do 3 miles of boulevard driving at 40-45 mph, she will start to stumble. Idle is OK but trying to get through a stoplight is nearly impossible. When this starts, I can't get the engine to spin over 2000 rpm. If I pull over and kill the engine, wait for 2 minutes, it starts right up and takes off like nothing ever happened. I'm in Arizona and it is hot out but the engine has hardly had time to heat up.

It seems sort of fuel starved but I would think that it would be more of a problem for the freeway driving.

What do you think?

Wirelessrat

Is it throwing any codes?

RexS

No "check engine" lights at all.

DanielRicany

Make sure all your gauges are good. Pull your MAF sensor and make sure it isn't damaged, pull out your o2 sensor and make sure it is not black or rusted. Test your fuel pressure when you first start it, and again when it acts up. Feel the coil when the problem happens and make sure it isn't hot.

Let me know your results.

rli7275

Get it on a scanner and have them look at the Block Learn PIDS. Anything that is out will have an effect on those values. Codes or not they will give you a direction to go.

MrWillys

I would put a fuel pressure gauge on it and check how fast it leaks down. You also need a scanner, or a cable to a laptop for diagnosis. Either injectors are, or a device is reporting incorrect information to the ECM. This is assuming nothing is mechanically wrong.

RexS

I'm thinking that the fuel system is probably delivering full pressure. It starts well and blows down the freeway without hesitation (I've already had a police car chirp at me). the Mass Air Flow sensor --hmm maybe, when they fail, is it intermittent? The function of an O2 sensor is to inform the computer that the fuel mix is fat it can slightly lean out the fuel mixture. A hot coil? I should add that the stumbling occurs within 2 or 3 miles of a cold start. Even in Arizona at 110 degrees, the coolant and oil are not at operating temperature when this occurs.

I'm thinking it may be the EGR. Perhaps it is sticking. The function of the EGR is to dump inert material into the intake when the fuel mix is very lean. When I'm going through the gears under 45 mph, the EGR should be cycling a lot. Maybe it hangs up. I also see from the wiring diagram that it has a computer controlled valve.

I'll try clamping the vacuum line to the EGR and see if this affects the problem.

rli7275

That symptom sounds like clogged injectors. As the engine warms up and goes into closed loop the ECU takes more control of the mixture. The adjustments it makes on a warming engine sometime due to clogged injectors will cause a drivability issue, that later fully warmed won't be as noticeable. Integrator value while driving may show it (snap shot) then an injector pressure drop/volume drop test.

RexS

This car had been sitting for 4 years. I know that has a major effect on the fuel system and it could be injectors but if wouldn't an injector problem show itself at other times.

I'll add that when I take the old girl down the freeway, I can drive her to the speed limit +5 (80 mph in AZ) pull off on an exit ramp, go through 2 stop lights, hop back on the freeway and blast up to 80 again without any hesitation. I've got some stopped time there, some low speed driving and an acceleration up the entrance ramp. If I had an injector problem wouldn't you think that it would show up there?

rli7275

Depends. Sometime a clogged injector will show up as the engine warms but on a fully warmed up engine it's not as or even noticeable at all. The ECT sensor voltage will cause the ECU to determine a pulse with that won't be sufficient with a clogged injector. Depending on the scanner and the sample speed and data stream you may even see it on the O2 sensor. That is why a scanner is so important to good diagnostics. The BLM and Integrator what we now call LTFT and STFT in OBDII systems are the best window as to what the management system is doing and the other data will tell you why. Your system is not OBDII and they were not as user friendly as the newer systems but they still will give you the needed data to fix the problem.

FostersPerformance

Try cleaning the MAF......

rli7275

That's a good idea. Quick and easy you may even see that it's crudded up. Certainly could cause the problem.

smurf5995

Adding my two cents here since I am having the same problem. When my engine is below 170 degrees, the car runs like a bat outta hell...but once above that, the car begins to, what I call, stumble, upon acceleration. This can be from a dead stop to cruising and giving it just a little more gas. The stumble almost seems like a hesitation caused my a plug or gas issue...maybe vacuum. However, I checked for vacuum leaks and there appear to be none. In fact, I went around the whole car and replaced every bit of hose under the hood, and where there were no clamps, I made sure to install one. I did have a bad MAF, so I replaced that as well as the two relays. I have also changed out the TPI, new plug wires, new plugs, rebuilt the distributor (all of these were original anyway). I also had a mechanic friend check the fuel pressure when the car was hot, as well as when it was cold. Pressure is fine. Therefore, this weekend I am taking the plenum off and checking the injectors and I might as well replace the egr valve since it looks original. At this point, I am at a loss for what the problem might be. I will try to remember to update you if I solve my issue, as it sounds like we may be dealing with the same problem. Just remembered, the car idles great at all times, when cruising the car operates great, and I have checked the timing and all seems to be fine. The voltage going to the injectors are within tolerance, and each plug is getting spark. I believe I have covered just about everything with the exceptions of clogged injectors or EPROM or ECM....but I have been told these would not cause it. But who knows.

Cliff Harris

You can disable the EGR by unplugging the EGR solenoid. Mine's mounted on the thermostat housing -- yours may be somewhere else.

I had a stumble off idle. It turned out to be a vacuum leak. They can be very subtle. In my case it only leaked when the engine was hot. It was the rubber hose between the PCV pipe and the intake manifold. It got pliable when hot and leaked vacuum.

Weird symptoms... ;-) Possibly an ignition module with no thermal grease or somebody put dielectric grease (WRONG STUFF!) under it. It might be a problem with the CTS (Coolant Temperature Sensor). The ECM uses the temperature reading for just about everything it does. If that's off/broken/bad connection/shorted/whatever you'll get strange behavior. Note that it's the CTS on the front of the engine that the ECM uses, not the one in the head.

rli7275

The need for a decent scanner with good Data Stream capabilities cannot be understated. Generally good used professional grade scanners from OTC,Snap On,maybe a GM factory,can be found on Ebay for not a lot of money. Pro's are always upgrading there equipment and the tool guy's don't give a lot on trade in(been there many times) I've literally spent thousands on scanners and software. A good scanner that is updated as far as you need it for your car and a little training not only would make working on the car you love to drive easier it's pretty rewarding to be able to master that part of the inner workings of your Corvette.

RexS

I'll try pulling the feed to the EGR solenoid and report back if any changes. I'll also clean up the MAF..it probably needs it anyway. I've been looking for vacuum problems but haven't seen the telltale cracking and loose connections of old hoses.

I found a Crane Cam H6 CDI box on the car. Now I have some reading to do to find out how that works.

I'd really like an old scanner and I plan to keep this car for a while. I'll start looking for one.

It is a great hobby.

bob guzzy

I have been fighting a problem like this for the last month or so on my 90, it was the O rings around the new injectors that were not sealing.

I went the changing sensor route also and nothing worked, I removed the plenum and put silicone injector seals on my new injectors that I put in at the start looking for the problem I got when I bought this car.

The TPS has a big effect on how the car runs, if it's not right the car will feel like an old quadrajet car that has a failing needle and seat and a float problem.

My 90 TPS like to be a .50v(key on meter reading the blue center wire ground anyplace it works the same as grounding the other wires) also check that the TPS doesn't have any dead spots hook up your volt meter and open the throttle and watch the meter it should rise as you open the throttle more if it drops the TPS is bad.

RexS

No codes? It turns out that the check engine light was burned out!

So I opened up the "information center" and swapped the "full power" light bulb (I have the L98) with the darkened check engine light.

Of course, I disconnected the battery so I lost any codes. I tried to pull codes and got 12,12,12.

I took it for a spin up I-17 and she took off like a bat out of hades. I took it about 20 miles up to "Bloody Basin Road" and she started to buck. I could not keep her above 65 MPH (idling in 6th gear). I pulled off and killed the engine. She restarted without hesitation and took off down the freeway... zero to 80 mph without hestitation. About half way home she started missing above 2K RPM. You can still keep her at 65mph with less than 2000 rpm. If I stomped on the accelerator, it would sort of bog down. Too much air and not enough fuel??

I killed her and restarted. She took off.

It seems like something is not reporting a good number to the computer. TPS?

Is this the