On my 1994 automatic lt1 c4 Both of my exhaust manifolds were glowing after only a 10 minute drive the other day. It is also very sluggish under load and doesnt seem to run very happily, but idles amazing and can also cruise just fine. But even leaving a stop sing or red light it feels sluggish. It has literally 0 codes from the paperclip diag. Im not sure where to start, i already checked all the spark plug wires, my cats are gutted so i know they arent clogged, i plan on test fuel pressure soon. Please let me know if you can help me at all or have had a similar issue, any help is appreciated

 

Extreme lean condition if the cats aren't there, check FP and ohm the injectors to start, a datalogger would probably tell you what the ECM is seeing to lean it out. Possible bad vac leak.

 

could be to lean or too rich. check spark plugs

 

I only pulled a few but they all looked about the same as this one, however i havent ran it much since it started running bad just 2 or 3 small drives

 

Check fuse #7 VALET. (This is power to the transmission. If this fuse is blown, the trans is in 3rd gear only. Sluggish performance for sure, I'm not so sure about the high EGT.)

That spark plug condition is not 'normal'. Look at the ground strap. There is no 'timing mark'. I have no idea what can occur in an opti car to change the timing mechanically, so I think the plug condition is a result of the operating conditions. Such as super lean, or high EGT from heavy loading from 3rd gear only. I'm not sure.

"No codes using the paper clip method". Which cavities did you jump? Did you use the speedo for the read-out? Ie, did you use 4&12, the onboard diagnostics? (Blown fuse #7 sets 4 codes in the ECM all in the low-80s. Something like 81, 83, 84, 85 I didn't look them up.)

Having said all that, after checking for 3rd gear only, checking fuel pressure while driving is a good place to begin.

 

Glowing manifolds on a normally aspirated street car mean only one thing, which is that the fuel air mixture is still burning in the exhaust upon the exhaust valves opening.

How that happens... that's also simple, the combustion process is not timed properly. It's a combination of spark timing, and combustion speed. More on this later.

First let's understand the key for tuning any combustion engine properly, the goal being peak combustion pressure at let's say between 5-15 degrees after top dead center. This about where the most power and efficiency is extracted. Simple theory, but expensive and time consuming to test. This is how the highest levels of motorsports are developed, and using pressure transducers installed in the cylinder head. Other means of testing are on a load cell dyno (eddy current brake or water brake controlled as an inertia dyno is useless here). If that makes sense, let's go to the next step.

What determines combustion speed?

Air-Fuel Ratio -- Let's say with gasoline, you may have the most complete combustion of oxygen and fuel at 14.7 parts air to 1 part fuel, very basic stuff. Combustion speed may increase slightly if you add more fuel due to the increased density of molecules in the combustion chamber, but after that it starts to decrease as there is not enough air to support the fastest speed.

Fuel atomization -- Depends on the health of the injectors, engine temperature, type of fuel, and nuances within an engine that brings the fuel out of the vapor suspension into droplets. The more the fuel is atomized, the faster the combustion speed. Point here is it's possible for poor atomization to slow it down enough to keep burning in the exhaust. If the injector spray pattern is poor, or worse it's a stream, that's an example.

Type of fuel -- they have different flame speeds, but since your car isn't tuned for methanol or nitro, we'll move on,

Heat -- the heat of the air and combustion chamber surface speeds up combustion, so the engine management system usually trims spark timing looking at the coolant temps, also cylinder temps on some more modern engines. If the engine is too cool, or if the sensor is bad (different sensor than used for the gauge, gauge typically reads the back of the Bank 2 cylinder head whereas the computer reads the front of the intake manifold).

Manifold pressure -- this is perhaps the most significant factor, and not always measured directly. When the engine is at idle, the TB is mostly closed, so there's a fairly high vacuum, maning very little pressure in the manifold. so the spark advance may be as much as 40-50 degrees of crank rotation before the piston reaches the top. Combustion speed is slow, and even though peak pressure timing is not as critical, it may not be correct for complete combustion prior to the exhaust valve opening. Where this comes in, is tuning or mapping the spark table -- 3D table with one axis RPM, the other axis either air flow meter - manifold pressure - or throttle position... and the resulting grid is filled with spark timing advance numbers. Such as 18 degrees BTDC for cranking RPM and zero throttle, or 35 degrees advanced for full throttle 5K RPM, or 45 degrees for high RPM and no throttle. Those are big swings, and they all adjust for the air pressure, working together with the fuel table. Note that the exhaust system installed may also change this formula.

So this is how the engine manages combustion timing or peak pressure timing, which relates to the extent to which fuel air mixture continues to burn inside the exhaust causing the glowing. Keep in mind that if the cam timing is off or too advanced, the exhaust valve can open a little bit earlier. Also if there's an exhaust leak, additional air is drawn into the combustion process, and also supplied to cause continuation of combustion in the exhaust system, even if the leak is 30-40 inches away from the exhaust port... and this can also influence the O2 sensor to add more fuel which slows down the combustion speed which also adds more fuel to burn in the exhaust with more oxygen there to burn it, etc.

Hope that helps, we may be able to guess the issue "it's too lean" or "dirty injectors" or "faulty MAP sensor" etc. but it's up to you to apply the logic to trouble shoot and find the cause... or advance the spark timing (assuming it's correcly calibrated as measured with a timing light) to band aid whatever is slowing down the combustion.

 

I will check on that fuse once i get home but i dont think its stuck in third gear.
i also think i did the paperclip diag correctly becuase the only code i got was c12.
i will also just replace all the plugs if this one does not look good because they all looked pretty much like that one
i will also check the fuel pressure and check the fuel filter

 

[QUOTE=danny polo green:

I will check on that fuse once i get home but i dont think its stuck in third gear.
[color=#2980b9]Ok, cool. 3rd gear starts will definitely cause sluggish performance. It was just a thought as you do have a 94 with the electronic transmission.

i also think i did the paperclip diag correctly becuase the only code i got was c12.
Yep. C12 is the Module 1 "I'm in diagnostic mode" message. If all you saw was C12 then "- - -" 4 times, that is indeed "no codes".

i will also just replace all the plugs if this one does not look good because they all looked pretty much like that one
The plugs are probably ok. I don't see anything physically wrong with it. Just the timing mark and evidence that it was in a lean or 'hot' running chamber.

i will also check the fuel pressure and check the fuel filter.
Definitely.

 

"Checking the fuel filter" is a lot easier said than done on any LT1 car. The filter is very fussy to access and change. Absolutely must be done with a cold exhaust system as the cat is right there. If you're going to "check it", have another one on hand and change it.


However, when I replace fuel filters I do "check them" to assess whether they are the cause of the symptoms I'm chasing, or whether I left it in service too long. (I just recently saw in my FSM that there is no facotry-recommended change interval for the fuel filter. The FSM states "Change if it becomes restricted and is affecting performance.")

>> Blow into it in the direction of flow. If you can't blow through it, it is restricted. (Wipe the inlet off before touching it with your lips. Gas doesn't taste good.)
>> If you can blow through it, stop before all the content is evacuated. Place your thumb and forefinger over the inlet and outlet and shake the **** out of it. Place the inlet side down, and pour out the contents. Observe the color of the gas. If it is brown and nasty, that is dirt, it was doing its job. If it looks like gas out of the pump, the filter probably hasn't been in service very long. << In this case, if it passed the restriction test in step 1, and there isn't any dirt, I put it back on and save $15.

 

ok i am still stumped by this issue and cant figure it out. i checked the fuel pressure and it was all correct, should i test it on the road and see if its different under load? i also discovered that with the MAP sensor unplugged it ran much better so i replaced it but it did not fix any issues. i plan on checking out the egr valve to make sure that it is all working properly but that is just a shot in the dark. i am not sure where to go from here.

 

Clogged cats?

 

@AZSP33D gave you a very nice write up that I feel would really help figure this out.

Have you verified that your timing is correct? This should be step #1
If your timing is correct then if it were me I would start to look at the fuel system. So if the obvious things all check out I would pull the injectors and bench test them. This is fairly easy to do. There are lots of YouTube videos that show you how to do it. Use carb cleaner instead of gas during the test to clean them a little. I doubt this is your problem but doing this is almost free.

Does this happen at idle? If the manifolds will glow from idling then there isn't a reason to check the fuel pressure under load.