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Motor head!!!!
Feb 13, 2001
Valencia, CA,USA
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Last Tuesday my car would not run past 2,000 RPM

I limped it home
and found the center button on the distributer cap
has self destructed... I put a new cap & rotor in
Checked the timing.... 6 deg before TDC
and the car ran... better

Now it has a flat spot just off idle
so I changed the Throttle position sensor and set it to .550 Volts and cleaned the IAC valve ( boy what a mess in there YEECH!) I took the throttle body off & the plennum off... cleaning it off as I went ( found a lot of extra hardware while I was down there....Bubba musta been here)
Checked the EGR and it had been replaced not to long ago, that was all cool, I put it all back together.

Drove it around the block and things were not good!
The thing now pings like a bandit and still has the flat spot off idle.

Did I just get a bad load of gas and not know it???
( No chalupa jokes here...)

The car is running @ 225 deg ( Normal )
Oil is Castrol 20-50
Oil pressure 33-40 at idle
I run 92 Shell or Chevron
No codes ( cept 12 )
While I was doing the maint I had the battery off
and the computer did see the removal of the connector to check timing ( Service engine soon )
and I did clear it!

I am suspect of the spark timing control module
That should retard the timing if it hears a noise right? The milage is about 12 mpg not good!

Enging has 116K on it... and really well maintained since 104K when I bought it.

Any cosmic thought???
Just because the EGR is new looking, don't rule it out. The valve can be bad and not always set a code. A bad valve will make that car drive terrible.

Check your G.M. repair manual for the full diagnostic technique. Replace it with only a G.M. part to avoid other problems.

Bad plug wires, and or plugs will do the same thing--and won't set a code until the condition gets too bad.

Get that 20W/50 oil out and put something thinner in like 10W/40 or 30.

A bad EST will set a code.
No, no, no, no, no

Okay, I am driving home from work and start thinking about your problem. Ping, ping, stall. Hmmmm. Yes the EGR can cause all kinds of problems--I am guessing yours is................................................Low Fuel Pressure.

That won't always set a code either, and cause the car to run terrible. There are three main things to consider. The fuel pressure regulator, fuel filter, and fuel pump. As bad as your problem is, my money is on the pump. You might as well change the other things while you are at it.

You will need to get a fuel pressure gauge that reads up to at least 75 psi. Check your G.M. manual for the exact pressure that your year model should have on the rail. There is a simple way of bumping the key, starter, and starting the car and watching the different pressure readings. I hate to go from memory, because it is usually wrong-however, you should see it spike up just around 45-50 when you turn the key, and it should hold around 40 psi running. Don't quote me on that.

The pump is EASY to change in your year model. Check the vendors here--always get either a G.M. pump, or a direct replacement.

Chris McDonald said:
Okay, I am driving home from work and start thinking about your problem.

This is what the Corvette Action Center COMMUNITY is all about! Thanks Chris. :upthumbs
Testing EGR/ IAC question

Thanks for the info.... more Info
with Mighty Vac in hand.....

When you have the car running and pull a vac on the EGR line, the idle will faulter, you let it go it clears up.

Car off, Plenum out, EGR in hand pull VAC. watch Diaphram pull back & hold not drain back. As far as I'm concerned...it passed the test. I could tell The EGR was Newer... the cad plating hadn't worn off the bolts yet.. and the rest of the valley WAS SLIMED.. what a pain to get the back bolt by the runners.

I changed my Distributor today because it felt like about 6-10 degrees of slop (1st thought it was mechanical advance... and said WAIT A darn minute)
I turned out the distrib gear was WORN bad... quite happy with my self... thought I nailed it... set timing 6 deg BTDC.. with the connector out, pulled battery to reset ( 5 min )

On the freeway it cruises great until you romp on it and it sounds like Carmen Miranda's rythm section... ping -ping . Still I had the same problem!!!!!

And it still has a flat spot off idle
So Chris I agree the next thing out is the fuel pump (changed filter Wednesday) or at least measure it while running... could be the regulator though??!
Now the IAC question....
How much travel should there be on the pintle?
mine feels like about 1/32 of an inch should it compress more than that? I would think so maybe 1/8th or more... it had a bunch of carbon or soot on the pintle shaft but I resisted the urge to spray WD-40 and work it loose. and the car idles like a big block with a hugh cam.....

Thats all for today....
Re: Testing EGR/ IAC question

vigman said:
...cruises great until you romp on it and it sounds like Carmen Miranda's rythm section

ROTFLMAO, I'm sorry Mike, I know this isn't a joke to you, but that is so funny. :L


What is so strange about your problem is that it is not causing any fault codes to trip. Otherwise, the description of problems you are having sounds exactly like a bad Mass Air Flow. However, no trouble code, computer is working, - rule out the mass air flow.

I am glad to hear you went the distance on the EGR valve. Very few people do and will replace everything-but that. Sometimes when they do replace it, they get a cheap aftermarket that requires washers to be installed to make the proper orphis.

A bad regulator will give you trouble. You can get a replacement diaphram pretty cheap for that. Not a bad idea anyway since you have 100k plus miles.

**************FINAL THOUGHT**************

Any chance you have a bad CAT? A bad converter won't make you run lean, but it sure will cause a dead spot in the acceleration curve and cause the engine not to breath in the higher RPM. This could cause it to pop and kick since it is trying to take in more air than it can exhaust. They will also cause POOR GAS MILEAGE when broken up inside. (sounds like we are on the right track now) There is no engine code for a bad CAT other than any symptoms it may cause.

There are a couple of ways to check it. Start with a vacuum gauge on the engine. Then, to confirm your finding drop the rear y pipe and take a visual up the center cat. The Pre-cats (if you have them) can't be seen inside. You can however, pound them from below with your fist and listen for any noise inside.

With 100K plus miles, you obviously have a combination of items that need attention.

Hope you fix it.
Re: Strange

Chris McDonald said:
Any chance you have a bad CAT?

Good catch, Chris. When my cats went south, my Vette wouldn't get out of it's own way! Actually, if they were noisy (to me it sounded like bad rods!), the car ran fine, but if it got quiet it simply had no power. They would plug the exhaust passage, akin to stuffing a potato in your tailpipe. ;)

Here Kitty....

BZZZZZZT on the CAT answer

Had them re done 2 months ago when Ugo's were passin me...

It's got power on the big end (but it pings)

It idles like....( Hmm what's that smell??)

For the pinging...I'm thinking to run some fuel cleaner... maybe to much carbon on the top of the heads... based on what I saw in the plenum I must be 11 to 1 compression.. I will get a fuel pressure gauge and see if it a reg problem.... but if I was running to lean... wouldn't I throw a code there too!
Common guys...if it were easy we wouldn't be spending all this quality time

And for the final twister

I got off work @ 11:00pm jumped on Freeway romped it... full power and just VERY slight ping
could the incoming air temp make THAT much difference? Still No codes.. I'm going to change my O2 sensor tomorrow just for grins ( don't know when it was done last)... so that's the news!

Ping Only?


So what your are saying is the car now runs fine, just pings when you thrash it, or when you just start to ease on the throttle?

I was under the impression it was also running terrible.

Well, it is summer time, and the hotter it is, the more it will ping. That includes the outside air temperature. If you replace any sensor, replace the knock sensor. Follow the instructions and make sure you don't get any thread sealer that caused the sensor to be less accurate.

I am guessing you have the cast iron heads. They don't disapate heat as well and will ping more when hot.

Go back and check the spark plugs-not only that the gap is correct, but that they are the right heat range.

Follow up on the fuel pressure.

Other than that, sounds like you have it figured out with carbon build up, and Summer time driving.

You might even try some octane boost and see what happens.

That smell must be from the fresh cats.

You replaced all three then I assume?

My old 85 use to ping on a regular basis when it had the stock engine. You may be very close to normal.

No Chris
My ( joke ) comment was
It Idles like ( Sh**) what's that smell, get IT

Aluminum heads.. Gaps correct..range correct

I think it's old motor, carboned, got a bad load of fuel, Bad IAC blues time.

So I'm going to give it a rest today... will continue the battle monday

Pay Attention


I should pay better attention to things. I thought your 1988 was a 1986--and thus......you guessed it..........cast iron heads.

Alright, well I am fresh out of ideas until you check the fuel pressure. You know though, you can have good fuel pressure and a bad injector. It still won't set any codes unless there is an excessive situation. In other words, one or two cylinders could be weak/lean on the opposite side of th engine from the O2 sensor.

I am with you. Did all the car work I care to for one weekend. Time for some rest and back at it next week.
If I'm reading this issue correct, the original problem was the distributer rotor problem, replaced/retimed. I had an 86 that seemed to have timing problems but was right on the mark. The problem was the harmonic balancer. The elastomer was failing, and only after it got so bad that it was rubbing on the timing chain cover did I realize what was wrong. Over time the outside, with the marks, was actually out of position with the crank. New balancer, re-time, all was better.

Dennis :beer


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