I have a 1985 coupe, ( should still be listed below?) It has est 47lbs of fuel pressure at gauge on rail, air flow is great, throttle plate and linkage all works, fresh and operational O2 and M.A.F. sensors, steady idle, and just to be clear it is full of oil. For the most part it will drive fine untill W.O.T. then check engine light comes on (which I will get the code for monday in the daylight) and it looses all throttle response. So after that I get back to the garage and the manifolds are glowing red hot. Indicating running very rich? Could this be related to HEI super coil being installed recently ( 1,500k ago?) Thanks for any and all help.

 

Sounds like you might have an exhaust obstruction causing the heat buildup cat converter possibly

 

I second the exhaust problem. The cats probably plugged or melted. Pull the cat off and try running the car. Make sure its not running too rich and there are no codes. You don't want to buy a new cat and then melt in again if the engine is running too rich.

 

If your manifolds a glowing you are not running rich your lean .Check your timing

 

Yup yup, very lean or very restricted exhaust. I think you would know if the cats were clogged up or you wouldn't have any power. Check the code and I bet it will say you're running lean. A clogged cat usually doesn't set a code.

 

Was the distributor removed before you had this problem? Timing will make them glow cherry red.

 

Yup. Sometimes you can hear some noise being caused by restriction. Listen to one side vs the other and see if you hear a difference.

 

Ummhmm, either a clogged cat, VERY out of whack timing, or very lean condition. Since you throw codes at WOT and not at part throttle I would guess that your open loop PE mode might not be getting enough fuel into your engine, clogged injectors maybe? Pull the codes that are being set when you go WOT.

 

i say your timing is off or your exhaust is clogged up in the precats or main

 

I would guess clogged cats. The performance and sound change is gradual so you won't notice much. Will your car rev throughout the entire RPM range? Clogged cats should restrict the RPM range. Turn on the car and feel your exhaust flow. If it feels weak/and or cool your cats are clogged. Good luck.


[Modified by LT5FIRE, 5:16 PM 1/21/2002]

 

You can test for clogged cats by using a vacuum gauge. Connect the gauge to manifold vacuum, raise rpms up to 2000, watch vacuum gauge. It should stay at a steady reading, if your cats clogged than vacuum will drop. As the exhaust gets restricted it keeps the engine from building proper vacuum. I would try this test with the car fully warmed up.

good luck!!!

 

Did you try drilling a hole in the manifold to "let the pressure out" :jester

 

Pete...dead wrong. Only fuel burning in the manifold will make it glow red. Nothing else will, PERIOD.

I would check basic stuff, check plug wires, check spark plug electrodes for condition, check fuel pressure bleed down ( bad injector even could cause it ).
Bad valves/ bad valve springs will also cause it.

I've seen a lot of causes of glowing manifolds/headers, all of them were some item causing unburnt fuel to reach the exhaust system.

Heck, I've even seen a bad knock sensor causing limp in, which makes everything run rich and glows the hell out of the manifolds.

Even turning you timing up can cause it, since it leads to spark knock which triggers the knock sensor.

It IS caused by burnign FUEL in the exhaust, make no doubt about that.

 

Had another car do this on time.the gear on the bottom of the distribuer shaft was slipping,pin had sheared,causing engine to get out of time.Manifolds would get so hot they looked transparent.A 50 cent pin cost be lots of money :yesnod:

 

I guess all the people who have had the exact same symptoms out of a clogged cat were just imagaining it?

 

Well I'm gonna throw in my "guess" with the rest of'em.

The fuel filter is clogging up at high flow rates or the fuel pump is crapping out at high flow rates.

Either will cause a LEAN condition. (It difinitely not running out of AIR).
If there were enough fuel in the exhaust manifolds to burn you'd be leaving a black smoke trail behind that you could see in your rear view mirror.

For some reason the fuel mixture is getting excessively LEAN (NOT RICH)
RICH will cool everything down and hence no cherry red exhaust manifolds.
The more fuel you dump in there the cooler the piston tops,valve heads and seats AND exhaust manifolds will become.

With a clogged CAT you'd be so down on performance it would be hard to obtain any speed.
Sounds like it's running about 100 degrees LEAN of PHT. (Peak Exhaust Temperature)

Any leaner than that and the exhaust will start to cool down again.

RICH fuel mixtures DON'T cause high exhaust temperatures.
(I'm not up to fighting over this with anyone here, I'm just trying to help)

First check your exhaust to see if there is a restriction in flow as a lot of the guys here have suggested.
If the CAT is bad then MAKE NO MORE TEST RUNS!

If it's (the cat) ok then see if you can tape your fuel pressure gauge to the windshield and do just one more run to check your high speed fuel pressure.

(Don't keep making test runs because if the manifolds are cherry red when you stop then you are VERY close to "meltdown" of piston heads,ring lands and valve edges.
Just plan on a short test run (about 1 minute) for diagnostic purposes.
You don't want to inflict damage on your engine while you're trying to help it.

I think at WOT the computer quits looking at the O2 sensor and goes to "max pulse width" on the injectors to get a much fuel as possible to match the wide open throttle body plates.
Anything less than WOT the O2 and TPS becomes the deciding factors for the ECM injector pulse width. (With a modifier from the MAP to adjust for barometric pressure changes)

BTW, your Vette uses the "batchfire" form of injector pulsing.
The injectors are wired in two parallel sets and the computer turns on four then the other four.
Later on GM went to the timed injector system which syncronized the injectors with the intake valve opening.

I guess the computer "could" be bad and cause this but I'd look for something more simple first. (like a plugged cat or fuel filter)
(Check the connections to the fuel pump...could be losing connection while running)

This is the "short course" on fuel injection. Hope it helps.
Post here to let us "armchair engineers" know what you found.
Good luck, happy trails and all that stuff.

 

Thanks VetNutJim.I know I wasn't dead wrong.

 

If you can realize the fact that running rich will cause a converter to overheat, extended periods of it will melt and clog the cat.

If you could see the problem, not the symptom.

 

WRONG

The mere fact that the header/manifold is getting red means most of the fuel is burning, and the rest will burn in the converter. You will see little or no smoke unless it is WAY TOO rich or you have a burnt up converter already.

Folks, a converter can only take so much heat. A converter does not convert Oxygen to anything...it converts hydrocarbons ( read: unburnt fuel ) to water/hydrogen with a few other byproducts.

In otherwords, the only way a converter will work is with unburnt fuel...which is what is was DESIGNED to do. ( In otherwords, too lean will NOT hurt any converter, and that is FACT, no guessing here. )

 

BBA, you seem like a crabby old guy like me, I really like that! But I don't think you are old, so you just a crabby young guy - but that is a good start. :D

Just to make you crabbier (is that a word, don't care I'm too crabby): GM does provide extra fuel under heavy load to cool the cats! Cause a rich burn is a cooler burn, a lean burn is hotter.

But you are right in that a constant rich mix will kill the cats quick, there is no question. However I think, and I am asking, that this is because the burn is so dirty - not because it overheats the cats.

Likewise, and again I believe, a constant lean burn will be very hot and cause hot exhaust gases which can also decrease the life of the converter.

In my old cars with carbs, the classic cherry exhaust manifold is timing - usually way off. I have also seen shops use exhaust gas temp measurement to diagnose lean cylinders - hot!

So as grumpy as you and I like to be, there is a lot of area to work around here. You are right, and they are right.

I encourage you to carry on in the crabby tradition, I like it!

Best,

Your crabby friend, Pete