dr309

I have a 1990 corvette with a manual 6 speed that started having an issue, I have looked at several things but I cannot figure it out. The check engine light doesn't come on but does work, the idle is anywhere from 1200 to 1500 rpm with car moving a few mph, If sitting still it is around 900 to 1000 rpm, very hard to start, have to put pedal to floor to start, black smoke sometimes at startup, jerks on take off. Just started doing this. Any help would be appreciated.

calvinlc

Have you looked at your TPS sensor for proper functioning?

dr309

I have looked into that last night, it seems to function ok. I don't believe I have a MAF sensor but I noticed the plastic piece that connects the air intake to the filter was loose from the air intake. I reconnected it but I am wondering if dirt or something could have gotten into the air intake and clogged up something? Could this cause this problem? I have looked for vacuum leaks but haven't found any. I rechecked for codes but didn't have any.

96GS#007

'90 doesn't have a MAF sensor. Black smoke typically means fuel. Check your fuel pressure regulator. Pull the vacuum line off. If it has fuel in it, the diaphragm ruptured. Not unusual given the age of these cars.

dr309

I will check this out tonight and post results.

JimLentz

Check for vacuum leaks.

dr309

I have checked the fuel pressure regulator for fuel in the line but found none. I think I am going to pull off the throttle body and clean it and check for leaks.

dr309

I have been looking for vacuum leaks but so far I have found nothing. I am going to try taking the throttle body apart and check for leaks. I will post what I find.

aklim

How good is your timing? Check that first. You should be about 6 degrees and make sure the balancer isn't coming apart. Check you scanner and see if you can achieve 20 counts on the IAC when it is at full operating temp and everything but the motor off. If it is higher, turn the screw in, rev and recheck. If it is lower, turn the screw out, rev and recheck. You want it to settle about 20 counts. If you are constantly at 0 even with the screw all the way out, you have unmetered air going in somewhere, either as an air leak or the IAC pintle is not closing all the way.

If you want to clean the TB, which given it's years, isn't a bad idea, you are going to need a gasket set. Take the top cover off, disconnect the coolant hoses, disconnect the TPS and remove it, disconnect the IAC and remove the TB. Take the IAC out and then remove the housing. Spray all over to clean anything black and the passages in the TB. Reassemble with fresh gaskets.

When you are satisfied with all that, hit the sensor the ECM sees for engine temp with an infra red thermometer and then check to see what the scanner sees for temps. They should be close

Also, put a fuel pressure gauge on the rail. See what it does when you turn it to run position. Pressure should go to 40-45 and hold for a while. Tape it to the glass, remove the hose from the fuel pressure regulator and block it up. Do a WOT run and see what pressure you get.

Check the plug wires for leaks. Spray a fine mist AROUND the wires at night and see if there is an electrical storm.

dr309

This is a lot to absorb not being a mechanic but I believe I can handle it after I get some equipment, timing light, fuel pressure gauge etc. I will get these things tonight and check these things. Thank you. I will post results!

aklim

I have never nor do I claim to be a mechanic. But I did stay at the Holiday Inn Express.

dr309

I am considering staying there the night before I start on this! Can't hurt!!!

aklim

Have some booze and a couple of hookers too.

dr309

That will be after I solve the problem.

dr309

The fuel pressure is at approximately 43 pounds, the timing is at 6° before, TPS is at .525, cleaned throttlebody and replaced the IAC, it was gummed up. I am still having this problem but I cannot find a vacuum leak anywhere. Are there any hoses inside the car or something I'm missing?

aklim

When? With everything running? How long does it hold? What about at WOT?

With the tan wire disconnected, right? Does the harmonic balancer look good?

Don't care. Does it rise smoothly? Is that what you see at the TPS or at the ECM?

Did you take the IAC housing off? Did you clean all the passages?

Before we go finding a leak that does not exist, what is the IAC count?

dr309

Can you tell me what WOT means? And I don't have a scanner to tell you what the IAC count is. I have it all back together and going for a test drive and see what the results are after a quick recheck. And yes with the tan wire disconnected. The fuel pressure held 43 pounds with engine off. The .525 was at the TPS. No I did not take the IAC housing off.

calvinlc

OK, did you check your TPS at wide open throttle as well and watch the voltage transition smoothly as you open your throttle? The TPS should start out at approx. 0.5 volts and go to about 4.6 volts at WOT. Also try checking your MAP sensor reading. With the key on but the car off, it should read 4.5 to 5.0 volts. With the car idling it should be about 2 to 3 volts. If you rev the car up in neutral and hold the rpm at 3k or so, it should read lower than your idle reading by a bit.

aklim

Wide Open Throttle. That will have the maximum use of fuel so we see how well it can recover when stressed to the max.

I'd get one. Borrow one for now form an Autozone or something. You are going to need proper tools or you are going to be guessing all day long.

If it holds like that for a couple of hours, that might be ok. However we still should know if there is a pump issue or not which is why I suggested we stress out the fuel system to the max with a WOT run.

Hopefully that is what the ECM sees. Never can be too certain. I prefer to work off what the ECM see as opposed to what you or I see.

Any reason why you thought the rest of the system will be clean? I do it as a 3 year deal while my injectors are removed to be sent for cleaning and testing. With the proper gaskets, I can have it in and out in under and hour. All cleaned and with fresh gaskets.

dr309

I would like to thank everyone for their expertise. I have cleaned the TB, replaced the IAC and TPS and adjusted timing. After a couple days of driving the car is performing like a new one. I am grateful to all for your input. I am not much of a mechanic my expertise is electronics but your instructions helped me fix this problem.