Frustration with 87 coupe - idle/fuel problems
 

Hello!

I need help...this is a continuing dilemma from last summer that we are starting to tackle again as weather permits.

Last summer, my 87 coupe blew a head gasket. My friend and I broke the entire top end of the engine down, and rebuilt and cleaned everything and we put things back together.

The car WILL run, but at a very poor idle, and sometimes not at all unless we coax the idle to stay at around 1000 to 1500 RPM. My friend describes the situation as sounding like a cylinder is not firing right/spark plug wire crossed.

Here is what we have done:

Broken entire top end down, replacing head gaskets, disassembling heads and cleaning everything, reassemble heads according to service manual, torque everything to specs in service manual, including setting valve lash per service manual. We also disassembled the original fuel injectors, replacing the filters and o-rings.

We have replace the IAC valve and even tried the trick of unplugging the idle control wire and resetting that.

Today, we reflected on our work and recalled that when reassembling the heads, etc last summer, we dropped one of the spark plugs on the cement. Thinking that perhaps a plus was damaged, we replaced all 8.

The problem persists - the car WILL start, but will not run without us manually manipulating the throttle to keep the RPM up...

The car has 199K miles. I am at the point that I cannot afford to put more money into a car that has such a low value, but don't want to be defeated by something simple that we are not thinking of.

Perhaps a fuel injector was damaged in reassembly?

The car ran fantastically BEFORE the head gasket blew...

We are at a loss as to how to proceed. Is there anyone in the St. Louis area particularly that could help us, or someone we could take the car to for help?

Any ideas would be appreciated.

A couple of questions:

Have you checked for vacuum leaks? These things are notorious for leaks;

Have you checked the minimum air setting? There is a specific procedure for this in the Helms shop manual.

I had a similar problem and found that my oil dipstick was leaking a BUNCH and then, once I set the minimum air, it runs lots better.

We did a bit more work just now:

Using a mechanic's stethoscope, we were able to verify each injector is working by listing for the ticking sound from each of them.

By minimum air setting, I assume you mean verifying the Idle Air Control valve function. We did that, and even replaced the unit.

Despite all of this work, we keep thinking something is wrong with the timing...

If RPMs are kept above 1000 to 1200, the car runs great, but WILL NOT run on its own below that. It will begin to backfire below 1000 RPM, eventually dying at about 500 RPM...

Anyone local in St. Louis that could possibly help us, or refer us to someone that might know what to do??

Do an Ohm test on each injector to see if they are good. Here is how:

You have to expose the terminals on each injector. Lift up on the spring clip and remove the connector. Then take an ohm meter and put a lead on each terminal. What you are doing is reading the resistance of the coil. The Ohm reading should be between 12-16 with all your injectors within .5 of each other. You cannot check the coil with the connector on.

Here is a How To video from Jon Banner:

http://www.youtube.com/watch?v=_g8PdrT0MCU

If I can hear the ticking sound from each injector with the stethoscope, doesn't that confirm that the injectors are good as they are firing?

I'm going to ask something for very stupid here to check, but is the distributor cap positioned/sitting correctly. Had an idle problem on mine when I tuned it up, found the dist. cap wasn't seated correctly. Just a suggestion...

We ASSUME it is on correctly, and that the distributor is bolted down right, etc...

We simply don't know what to do next...

Verify that the dist rotor points to #1 wire when timing mark is at 6 deg BTDC and that all wires are in the proper firing order.

Use a vacuum gauge to check manifold pressure at idle.

What is fuel pressure at idle with the FPR vacuum line disconnected and plugged? It should be around 44psi.

A scan tool will show if fuel/air is correct.

No, it does not confirm that they are working properly. Do the OHM check; its real simple to do and you don't have to dig into the engine or disassemble anything to check them out. Should take you less than 10 minutes to check the injector resistance.

Also, you need to see what your fuel rail pressure is and if the pressure disapates quickly after engine shut down. This would be a sign of a leaking injector(s).

I would also make sure that the rocker arms are adjusted properly and are not too tight. Over torqued rocker arms can lead to difficulty idling.

Finally, have you pulled the ECM error codes to see what the diagnostic system is telling you, if anything?

Let us know if you need help understanding how to complete any of these diagnostic checks.

I feel your pain--I recently took on the task of rebuilding an 87 convert with a toasted engine--radiator failed. As a middle-aged guy with some practical experience having grown up on a farm, I felt reasonable sure I could do this. I was ultimately successful in restoring the engine but the frustrations were overwhelming.

With one Corvette engine under my belt, I am hardly qualified to give advice but here are some of the things I learned [in no particular order of importance]: 1) this forum is great but you need a field service manual to navigate around the engine; 2) triple check the distributor post/gear and cap to ensure you have it aligned; 3); check all vacuum lines for leaks; 4) the cold start injector needs to be check; 5) so does the intake vacuum sensor [under air intake box and infront of distro cap]; 6) the mass air flow meter [between air intake and air filter] can be a problem; 7) also check that all the EGR components are properly re-connected; 8) if you removed the rockers, getting the values re-adjusted was a real challenge--for me; and 9) check the spark plugs, spark plug wires too.

My frustration was that after checking all this, the engine still would not idle correctly. The key was checking the display codes from the ECM [Engine control module]--there is a good discussion on how to do this on the forum. Turned out the ECM [$125] had gone bad while the car was under repair--who knows why but it did. Once you check all the obvious things mentioned above, you are left with doing diagnostics inside the ECM. Again the service manual helps but I finally went to a local hot-rod/Covette shop. Their experience pointed them to the ECM, where as, I might have pulled and replaced any number of sensors and parts, "hoping" to stumble on the root cause.

Bottomline--you really need mental health consultation before working on a car with 25 year old electronics like the Corvette. Good luck and God's speed to you!



-ki

Just checked all injectors with an ohmeter. Every one tests at 15 ohms.

Fuel pump was replaced about 1 year ago and only has a few hundred miles on it, so confident fuel pressure is fine.

Yes, your fuel pressure may be fine, but are the injectors leaking? You can't tell unless you put a fuel pressure gage on the rail and see if the injectors hold their pressure for at lease 30 minutes or so after you stop the engine. Check it out and see what you discover.

Also, have you pulled the codes to see if there are any error messages?

no codes stored?,sorry i read fast the thread ,but i bet it's a MAF related issue,bad idling it's related to a defective MAF, any 32-33 code?try a good one from a friend C4...

Re-adjust your valves. Nobody gets it right the first time.

Valves have been adjusted 3 times now.

We are going to wait for a good weather day and start over with setting the timing, reinstall distributor, and take the air plenum off and trace the routes of all of the vacuum lines.

If that isn't it, we will give up...

Some things that can cause rough idle:

Minimum idle adjustment:
http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
EGR stuck open.
Vacuum leak. Everybody misses the vacuum line on the bottom of the throttle body.
Timing not set correctly.

There are probably others I didn't think of...

Pull the codes and see what you can discover. If you need help on how to do this, let us know. :thumbs:

Many people don't get it right the second or third time either. Please don't take it as an insult, just personal experience. Unless you've done it enough times to do it with your eyes closed, don't assume they are correct.

I remember years ago a friend had a car that wouldn't start. We went though EVERYTHING 3 times with no love. Turns out it was just the valves.

Having the valves set too tight will create the exact symptoms you describe. That may not be it yet, but I wouldn't rule it out just because you've checked it before.

Did you set your base timing at 6 degrees with the timing connector disconnected?

I had a similar problem once. Here is a simple test.

Disconnect the MAF sensor plug. Try starting the car. Does it change how the car idles? If so then check the wires at the back of the distributor...so if they are in good shape.

Also, check for vacuum leaks around the runners, throttle body, or the intake manifold.


Are you sure that the wires are all connected to the correct spark plugs?