Hello all... first time poster here.

I just, I mean JUST bought a 1990 Corvette. It's totally sweet (my 1st Vette) but it seems to have a problem. Not a good start to my Vette Experience.

So, when it runs, the rpms rev up and down some 200 - 400 rpms consistantly. ESPECIALLY if the A/C is running. It gets so bad sometimes that it wont stay running. The idle is rough, the engine moves around quite a bit, not that smooth yet slight vibratiing that the LT1 is known for.

It misses consistantly through the rpm range. Almost feels like I'm dragging something the way it labors sometimes.

It failed DEQ with high Carbons. It's running a slight "blackness" to the exhaust.

So my question is WHAT is this? There are NO codes in the Computer.

PLEASE help,

DKOV -

 

Welcome,

Sounds like you need a good tune-up. Start with basics, Plugs, wire cap then adjust tpis for idle.

You may just need to clean the Throttle body.

 

Welcome to the forum!!! also with 85vet need a good tune-up. If you've got a 90 it should be a l98. Replace your fuel filter as part of the tune-up,and clean the IAC, that could help also. I'm sure you'll get lots of help here,enjoy your ride

 

I had a problem just like yours and I had to replace my fuel injecters.The new injecters fixed my problem.

 

I suggest you look at the injectors in your new vette. IF they are a dark gray color, you may have a problem with them. Check the resistance of each injector when cold and when the engine is hot. The resistance should be in the range of 16 to 17 ohms. If you find numbers lower than these, you have shorted injectors. Hope this is helpful.

 

Hook -up a vacuum guage and look for a steady reading. Also, run propane gas around your intake manifold gasket and check for a noticeable change in idle. You may have a vacuum leak. Also, check the voltage on your TPS. my butterflies were manualy adjusted and it made mine surge when coasting. I readjusted the TPS and the surging went away see my post from yesterday
http://forums.corvetteforum.com/showthread.php?t=863995
It says discovered my miss if the link doesn't work. I hope it helps
Mike

 

Tune-up (MSD 8.5mm wires, AC Delco Rapidfires, new STOCK cap & rotor), check TPS voltage, remove and clean the TB and IAC valve.

The blackness in the exhaust may be from the above, or from faulty injectors. Hook up a fuel pressure gauge to the rail, and shut the car off, the pressure should hold for quite awhile.

 

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Yes, very interesting but from what it looks like, the factory installed adjuster screw cover is still in place telling me the idle has NOT been messed with.

However, I did take the TPS off and notice that the throttle lever does hold the TPS open partially. Not all the back to the "start". I assume that is incorrect?

Anyway, I messed with the TPS and the ECU finally showed some codes. Due to messing with the TPS while the engine was running no doubt but it proves to me the ECU isn't bad. So I know that's not it now.

I reset the ECU by leaving the Neg Battery cable off for about 15 minutes and the codes were cleared and haven't came back but the lopey engine idle did.

I'm really leaning toward Ignition. It seems like it's acting exactly like an engine does with a plug wire grounding out against the block somewhere... although they are not. Does this sound familiar with your problem?

By the way... the car only has 42K original miles... still pretty new. This is not due to long, hard miles.

Thanks again and PLEASE keep the help coming

Scott "DKOV" Kovalik

 

Why did you emphasize "STOCK"? My car has the HEI coil on it. Could that be it?

Scott DKOV Kovalik
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Okay... some good news... and bad news

First the good news. As I posted before, I've been suspcious of an issue relating to Ignition for the lopey idle and such. I looked, again, at my timing and my distributor and took the top off again to look inside. THIS time, I noticed something I hadn't before.

Between the Coil and the Cap is a "Button". This button connects the coil output to the rotor. This cap is held down by a large rubber gromet so all you really see is the spring sticking up through. This time I removed the rubber gromet and low and behold...

The button had sheared in two. They were touching intermitantly and I'd bet the house that was my rough and lopey idle.

BUT...

Fixed it with a new Cap and new button and the lope has indeed gone away... or at least it seems to. But I cannot test it. Because there is something NEW wrong.

When I got it and backed out of the driveway for the test, I put it in 1st and stepped on the pedal. Not tire sqealing hard but get to traffic speed hard. It coughed and sputtered and even backfired into the intake!!!

This is new. And even worse than before. At least it would DRIVE before.

Still no codes either.

So I'm thinking I'd better check the alignment of the TPS since I messed with it following another thread's advice but can't see how I could have messed with it as the two screw holes don't allow for any alighment options and I didn't modify anything.

So I'm off to check out the MAF. Here's where the BIG suprise is... NOWHERE.

I can't find it. The book says its right there in the intake a the airbox. However, there is nothing plugged into my airbox. No wires, no sensors. None of the screen I hear mentioned. Nothing.

It's a K&N FIPK and appears to be aftermarket but still.

Correct me if I'm wrong but should there be a MAF right there at the beginning of the flex tube?

So, sitting here more puzzled than ever... in DESPERATE need of help.

Scott
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1990 does not have a MAF sensor.
It runs on a Speed Density system-MAP sensor.

I emphasize stock because usually the next question people ask is "would i get anything from a MSD distributor or Accel" when they are going to replace the HEI system, and with the stock intake, you wont.

 

take it out on the highway and put the hammer down and blow out all that carbon

 

Tried that... didn't work

Scott

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DKOV,

If you covered all the bases tune up-wise, it's time to check your 02/EGR/intake... 99% the prob is in one of these three if the problem still eludes you. Sucks to chase stuff, but thats what you're up against and any one of these will cause the problem you are describing..and injectors as Vader mentioned above..Again, I say this assuming you have ruled out plugs, wires and other common tune up items.. Good luck, let us know...

 

SAM LAM is giving you good advise, check the inj before you spend a fortune. Are the plugs fouled, any codes?

 

DKOV: When you replaced the rotor button inside the distributor, did you remove any wires from the distributor cap? If so, the correct firing order is 1,8,4,3,6,5,7 & 2. This starts from the #1 wire and go CLOCKWISE while looking from above the distributor. Sounds like you may have crossed two wires! Also, make sure you placed the rotor button correctly in the distributor. It is made to go on only one way, but I have seen this installed backwards??? Good luck and you seem to be learning a lot. Owning a Vette is a learning experience and a money pit! Suggest you buy a GM Helms manual and throw away whatever you are currently using. MAF on your vette....NFW....I agree with Vader!!!

 

Thanks all for the great advice. I've replaced all the plugs and wires so I know that's not the issue and they are all routed correctly.

I'm thinking that I had a combo problem the lease of which was the bad distributor "button". It seems that now, since I've replaced the button, I'm actually getting a code 22 - bad TPS. New on on order. News to follow.

HOWEVER... Bad news continues.

Taking the TPS out, the bottom screw was cross threaded when installed and I've managed to break the thing leaving have the screw IN the throttle body.

I'll need to remove the TB and drill out and retap the screw hole now. What a pain in the **** that will be.

Any suggestions on removing the TB and what steps I should take, problem areas to look out for and other tid bits of info not covered in a standard repair manual?

By the way, I'll need to do all this BEFORE I find out if the new Botton and the new TPS fixed my problems.


DKOV -

 

Okay brains... let's get to work here.

Finally got the TB off and the old screw end drilled out of it. Then tapped the new threads and installed the new TPS.

Keeping in mind that I have so far found:

-Broken (sheared) "button" linking the coil to the rotor in the Distributor
-ALL bad (and completely wrong) spark plugs
-Loose bolts holding the TB on the car
-Bad TPS (low voltage code)

and replaced:

Plugs
Wires
Distributor
Cap
Rotor
TB Gaskets
TPS

And did not find:
Any vacuum leaks
any broken hoses or tubes
no unplugged anythings...

And it's STILL doing exactly what it started out doing.

Idle surges. Especially under load.

What the heck. There's not enough space on this posting softward to wrinte down all the swear words I'm thinking right now.

For my first Vette... I'm thoroughly disappointed. It's no junker either. A near mint 40K original in perfect condition. What is up with this?




DKOV -

 

I think I may have found it.... please vuew this thread and respond if you can.

It appears my AC "On" Signal is not reaching the ECM. According to GM, this causes a bad and lopey idle even when the AC is off but especially bad with the AC on.

It shows the procedure for testing it and my test shows FAILED.

So... where is the "switch" or source of that signal?

HELP

DKOV -

 

Would you be referring to the AC on switch from the dash, or the AC pressure switch in the AC line? The pressure switch is really easy to find, the other probably requires taking the dash apart as it'll be part of the AC control head unit.