My 89 starts and runs perfectly when the engine is cold. After the engine warms up it starts to run very rough, feels like a misfire but not sure. It will not accelerate well. This started a few weeks ago and seems to now happen every time. No check engine light.

This is an original car purchased new, no mods and has had very few repairs, 33k miles.

I think I read somewhere that the engine control runs in open loop mode when cold and closed loop when warm. Might be a clue..

Any ideas??

 

Check MAF; I'm pretty sure that's what was wrong with mine. Once computer hit closed loop and was reading sensor things went screwy. Mine did not even throw a code.

 

I know this is an old thread, but, did you fix this problem? As i seem to have the same problem on my 89

 

Hello Damien89,

How old is your Corvette's Oxygen sensor? It is possible that your O2 sensor could be the problem. Before going too far check out the signal coming from your Coolant Temperature Sensor, they fail fairly often and can send erroneous (Incorrect) signals.

When the engine is running after first starting it is in "Open Loop", after it warms up it will transition to "Closed Loop" as the engine temp comes up. When in Closed Loop the engine uses three primary sensors to control the air/fuel ratio. The sensors are pretty straightforward.

The Oxygen Sensor (a.k.a. O2) reads the air fuel ratio and the signal oscillates back and forth from lean to rich very quickly. The signal should be between .1 and .9 volts and like I said earlier it is oscillating so fast it is hard to watch. As O2 sensors get old they start to get slower and the signal gets narrower. The Oxygen sensor is a device that should be replaced every few years. In the early days of FI they told us to change the O2 every 2 years or 50,000 miles. Your situation sounds like it might be the Oxygen sensor lagging behind in it's signal generation which really makes a mess of the air/fuel ratio. Another interesting thing is that when my O2 sensor failed I got no codes, they don't in the older engines. So the only way to test it is to use a scanner to allow you to see what is happening in real time. I suggest to my fellow Corvette Owners that they replace the O2 every few years. If it is on the car and the car is not used the O2 still wears out. There are a lot of problems with our Corvettes that could be solved with a fresh O2 sensor.

The Coolant Temperature Sensor (a.k.a. CTS) tells the engine what the current temperature is. It is very important as the temperature plays a major role in getting the proper fuel mixture based on the temperatures. Your engine starts okay and develops trouble in the closed loop mode. If the CTS were "bad" and it is sending a 300* temperature to the ECM when you try start the car it will be very hard to start. It will make the car very hard to start in the cooler months
If it fails the other way the engine will see a cold temperature and turn on the fuel enrichment needed to start the engine when it is cold and that will make your car act like the "Choke is stuck On". This situation "could" flood the engine when cranking it and will make the mileage go way down and you might even see Black Smoke from the exhaust.

Last but not least is the Mass Air Flow Sensor or MAP sensor. The MAF sensor on my 1988 C4 has a wire that is 1/10th of a millimeter in diameter that measures the air flowing by it. The MAF circuit also has a Burn-off relay and another relay to power the sensor. The purpose of the MAF sensor is to measure the Volume of air (in grams) going into the engine's combustion chamber. The Burn Off relay heats the little wire up and burns off any contaminants to ensure proper function. If the Burn Off relay fails the wire will not be cleaned every time you drive the car. This will lead to a MAF failure and they are expensive to replace. IF you ever replace the MAF be sure to replace the relays associated with it at the same time. If there is any leakage into the air heading into the engine then the MAF is not able to get an accurate measurement and this leads to trouble as well. The MAF starting to fail will Start to miss under a load like being in 6th gear going up a hill. My DD had one fail and it showed itself under loads primarily.

I hope that this helps you in some way. These fuel Injection systems are fairly straight forward. In your Corvette if you are experiencing Closed Loop issues this is where you should start. Does the car display any codes? Any lights staying on like the Check Engine Soon light? Take a look at the three sensors I described and see what is happening with them. If you have the Factory Service manuals then they describe how to test each one of these sensors. If you don't have the FSM then buy one, they are critical when owning a 30 plus year old Performance Car.

The one other thing you should verify is that your fuel pressure is within the prescribed range for your particular year. Fuel pressure issues are common on older C4's. Check the numbers to be "Sure" that the pressure is not aggravating any of the problems.

When your car is warmed up and you start it up does it start running poorly soon afterwards? Cold starting is okay it is just the transition period that brings on the problems, correct?

I am also available by PM if you need to get in touch. This "problem" with your Corvette will not last long.... With the amount of knowledge on the Corvette Forum somebody will know just what it is causing you problems with your Corvette.

Best regards,
Chris
P.S. I bought a used Snap On MT 2500 from a fellow Forum member and it is an awesome tool! If you have to find somebody with a OBD 1 scanner or something that will show you live data. That would make it fairly easy to determine the problem.

 

89 injectors are multecs and very problematic to short out when warm.
ohm your injectors warm. They should be 15.5-16.5 ohms.

 

Hi, wow thanks for the long informative reply!

the 02 sensor is still the original one that came from the factory,

yesterday i tested the cooalnt temp sensor and altough it looked like it was reading the temp as it should i'm not 100% sure it's reading perfectly,

i didn't take a look at the maf,

then i played a bit the fuel pressure regulator, while it was doing the symptom, i removed the vacuum line from the regulator and it seemed to appear to cure the symptom, without the vacuum line pressure went up to 50 psi idle, i'm not 100% sure the fpr is the culprit but it seemed to almost cure it while the vacuum line was disconnected.
I was thinking about changing the fpr just to be sure, I searched for a new fpr but they don't seem available anywhere, all i'm finding is rebuild kits. Can anyone point me to a new fpr? If i dont find one i'll have to rebuild the fpr. Apart from the rebuild kit, anything else i would need, gaskets etc while i have the plenum out?

 

Injectors were replaced 8 years ago, if injectors play would the fuel pressure behave in an erratic way, going up and down between 10-50psi multiple times every second?

 

Replace That O2 Sensor!!

If you are still using the original Oxygen sensor that must be a record! I was taught every two years or 50,000 miles you replace the O2 sensor. That will probably make a huge difference in the performance alone. Old O2 sensors really mess things up, I am anxious to hear how it works with a new O2 on it. They react very slowly with age and their signal is not as strong (accurate) any more. This is clearly a part that will have a HUGE effect on your performance and mileage.

Rebuilding the FPR with the kit is a very easy task, getting to "it" can be a bit more challenging. The gasket and spring kit is the "Standard" way of fixing it unless you are replacing it with a aftermarket FPR. Cleanliness is important.

I found my EGR vacuum solenoid was frozen and the vacuum line open to the air. I then had to replace the EGR as it was damaged by the EGR Vacuum solenoid not working properly. Having a leak in your vacuum might have a similar effect.

In the Factory Service Manuals there is a chart that shows the resistance value and the corresponding temperatures. It helps show you if your CTS is accurate.

Watch your pressure of the rails after shut down, if you have a leaky injector that will help point it out. When one of my injectors started leaking the pressure went down very quickly. Afterwards, it holds for quite a while. I use South Bay Injectors for my injectors over in New Jersey, they have some good deals. Normally the needle moves around but it should be stable after a couple seconds.

On some of these older C4's the wires on top of the intake manifold have become brittle and loose their insulation very easily. Having CLEAN battery connections is important for overall health of the Corvette.

I found that NAPA has a FelPro gasket set for the just the intake manifold gaskets. It had all the parts I needed to replace the FPR and the EGR on my 1988 C4.

Keep us posted...

Best regards,
Chris

 

Hi Chris,

thanks again for the reply,

Yes the car still has the oriinal oxygen sensor, it 'only' has 22000 miles on it but i'll change it with a new one, which brand do you suggest? Ac delco? Anything better?

As for the fpr yeah i know that getting to it involves removing the manifold, last i removed the manifold was to change the injectors.

After i shut down the engine the pressure drops to zero instantly. I'll have the injectors checked while they are out, but to say the truth it was always like this even when it had the original injectors.

Thanks again for the great help. i'll start ordering parts this weekend and have a go at it when they arrive.

 

Sounds to me like your fuel pump is going out, after it runs a while it starts pumping less.
Good Luck