After building my ride with Hot-cam, 1.6 rockers, etc, etc, I developed an off-idle miss especially, with the A/C on. ???? Plugs?, wires??, opti??
I have searched the car for months; replaced the plugs, wires, and re-tunned the chip, checked fuel pressure, injector resistance, etc, etc.... Well, I finally found the problem.
When you adjust your idle stop you will inadvertantly (or deliberately if you know about this already) change your TPS voltage and throttle response. In my case, I adjusted it out enough to disrupt the TPS sensor and create a hesitation/Miss off idle. Wide open runs were fine but, the off-idle miss was a mystery. To correct this; I found that you only need to drill or waller-out the TPS sensor holes enough to re-position the sensor back to it's original location (before you messed with it) while leaving the butterflies alone @ the higher, smooth idle setting. Walla! the off-idle miss went away immediately after adjusting the TPS. I played with a couple of different settings (I don't have a digital volt meter) and eventually, got it set close enough that the car comes off idle quick and crisp. SWEET, and as a added bonus, it doesn't surge as much when you coast either. Pretty good deal for 10 minutes work and zero cost. Definately Forum post material in my book. Anybody else have the problem? easy fix. I love this Forum!

 

how interesting... and how true...

good find!!!

 

great find!

...but i thought there was a way to have the ECM re-learn the minimum TPS voltage after you mess with it?

 

 

hmmm (staring at shop manual)... on the 92 it's manually adjusted by fiddling with it while monitoring TPS voltage with a scan tool or a multimeter.

there is, however, a TPS learn for the EBCM but requires a Tech 1 or T-100. it needs to also learn TPS at idle for proper ASR operation.

 

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 looked at it following this 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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Scott, what year is your Corvette?

 

He has a 90 with speed density and MAP, but no MAF as he'll learn in another thread he was in.

RUBYRED, nice find.

Oh yeah, Scott, I wonder if your timing may be off a bit now..if it had been set with that other condition..just a thought.

 

Skateparkdave...

I thought about that too. Problem is, my book doesn't say what the marks on the Timing gauge are. It is pretty rusty so I can't really see it that well.

I know that the large chevron is 0 degrees... that I can see from the book.

There are several notches to the passenger side of the 0 degree point and only 2 or three to the driver's side of the 0 degree point.

My information says I should be running it a 6 degrees negative but which notch is that? I assume that is one of the notches (or points) to the passenger side of the Zero point, but how many?

My big project now is to remove the Throttle Body and drill out the stupid screw that sheered off while removing the TPS. Just another thing to make this whole process MORE painful!!!

Your continued help is appreciated,

Scott

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Okay, so NOW I'm reading that not only does my 1990 NOT have a MAF but is may (or may not) have an ADJUSTABLE TPS.

Is this TRUE?

Scott

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glad you got it!!

It just mystifies me why people make adjustments on electrical components without the proper measurement tools or scan tools...

 

6? maybe? Not sure about your guage there.Maybe you should get a new guage thats not rusty.

TPS on 90 is not adjustable as far as I know. If it's bad you should get a code 21 or 22.

 

I just bought my 95 in Feb. It already had a Hot Cam kit installed. It too has a mysterious misfire/surge especially at low rpm cruising and part throttle acceleration. At WOT it runs smooth. I changed the wires, plugs, coil and air filter all of which needed to be replaced, to no improvement. I had just chalked it up to the fact that it still has a stock gear(6sp.) and the cam was just too big for stock heads and gear. I thing I will definitely try this fix, thanks for your post!

 

BUY yourself a digital voltmeter...They are not very costly and obtainable from any of your electronics stores...As someone else said in effect; it is CRUCIAL to use ACCURATE measuring devices such as this. Unless you have a bloody expensive analogue meter and skilled eyes as well, it'd be next to impossible to read the fractions of a volt needed to assess such measurements. Well, it would for ME anyway and a digital unit is SO much simpler.

 

Good to hear you found and fixed the problem.

 

Your distributor is off a tooth or your plug wires are off by a terminal or two....

I don't want to get off on a rant here...but...

Your Corvette is not your mama's grocery getter. To do the kinda work you're trying to do to your Vette (admirable by the way!) you need the proper equipment. No one ever timed an engine correctly by ear or set a TPS sensor correctly by eyeballing it! You have the repair manual, read the introduction section in the very front of the book. It describes the most basic tools that you will need to perform the repairs. A timing light, a digital voltmeter, a vacuum guage etc are just examples. These tools are not very expensive but are invaluable when making your Corvette perform the way it should.

Sounds to me like you have the desire and the basic knack for turning the wrenches. You're on the right track just slow down a little and do it right the first time. A wise old man once asked me...

"If you don't have the time and the money to do the job right the first time; where will you find the time and the money to do it over"?

 

Thanks for the good advice

My biggest issue is that I don't have a FACTORY repair manul yet and I've been chasing these issues with the help of a Haynes Manual (which leaves out all the unique differences of the 1990), the help I'm getting from this forum (which is sometimes about other years) and my own personal (and extensive I think) experience with other GM motors.

For instance, the book clearly says to check for the MAF sensor on my car and it goes over the TPS calibration for my car.

My car doesn't HAVE a MAF nor can you calibrate the FIXED TPS.

Those are a couple of things they SHOULD mention.

I do have a Volt Meter and Vacuum gauges and have been steadily working through the process of elimination.

In doing so, I've discovered a couple of things... Lots of loose bolts and such. The AC On Circuit is showing lower voltage than it should and....

Shazaam!

I have a couple of BAD INJECTORS!!!

At idle, they are not firing correctly but at WOT they begin to. Plus, they are apparently leaky. Steady drip, drip, drip of fuel.

That's CAN'T be good.

So I called a local Vette Expert and he says that despite the lack of information, the 1990 Injectors are NOTORIOUS for failing, get this, ELECTRICALLY. Not mechanically. The leak is just conincidence in his opinion but virtually every 1990 will loose the electrical stability in the Injectors right about 40K miles. Then he asked me how many miles I had...

Guess what? 43K miles.

New Injectors are on their way from Five O Motorstpors and we'll see.

I'm feeling... hopeful? Lucky? Dare I say Confident?

More to follow

DKOV -

 

How did we get from diagnosing a miss in an LT1 (something that I am interested in) to talking about a MAF from a 1990??

Start a different thread next time instead of hyjacking someone's topic.

 

Becaue my 1990 is missing in the exact same way and we're all sharing what we have found that solved the problem.

The MAF issue comes because of the suggestion that it's possibly a MAF sensor issue, which a 1990 doesn't have. That should help if another 90 owner is also trying to track down the same miss.

As I was when I first posted to this thread.

No hijacking here... try reading the entire thread next time before spouting off.





DKOV -