Way to much advance.

Hi, Name is Todd and thank you in advance for any responses to this problem I am having.

Jeep is a 1993 XJ 4.0L/AW4 4x4 with 231 t-case.

I recently replaced the engine and am having an issue with the exhaust manifold getting red hot on bank 2 (last three cylinders) when off idle. I first suspected a rich condition but have verified the fuel pressure regulator is working properly and O2 sensor is cycling the way it should. Suspecting the issue was in the timing, I checked ignition timing using a DRB 3 and found it to be 15 degrees advanced at idle and 46 degrees advanced at 2500 rpm. I have verified these number to be accurate with a timing light. This seems like WAY to much advance to me. The distributor is not adjustable (without removing the bolt ears). I am not sure that this is the correct distributor for this vehicle as I purchased it used with a 1995 motor in it. I do not suspect a plugged exhaust concern as my throttle response is good and have no misfires off idle. The casting number on the left rear of the block, which I am pretty sure is the date code, is 2280. I would assume this is 2/2/1980? The rebuilder of this engine put a 331 casting head on it. I am considering cutting the ears off of the distributor so I can adjust ignition timing and try and back it down.

Thank you for your input, it is greatly appreciated!

Todd

Some thoughts and wisdom for you.

If ignition timing were an issue, all cylinders would be affected.

A rich condition would not result in an overheating condition.

A lean condition would cause the symptoms you describe.

How to diagnose. Check upstream O2 sensor bank two voltage. Compare to bank one upstream voltage. It will likely come in low, indicating an intake manifold leak.

Also check that bank two injections are working properly.

I own a 2001 XJ, and can tell you that 10+ degrees of ignition advance at idle is normal.

Thank you for your response.

I agree with you in the fact that all cylinders would be affected if ignition timing were the issue. I should have worded my concern as both banks on the manifold glow starting with bank 2 and eventually turning into bank 1 as well.

The engine is not overheating, I am just glowing the exhaust manifold, hence the reason a rich condition would be suspected due to the unburned fuel (due to amount of advance) burning in the exhaust manifold.

1993 XJ only has 1 o2 sensor so I cannot compare bank 1 to bank 2. This is probably the reason the o2 sensor appears to be working properly. The unburned fuel exiting the cylinder head is being burnt pre-o2 sensor and mixing in the collector with bank 1 exhaust.

The exhaust manifold and intake manifold are both brand new along with the gasket. Checked and verified cylinder head mating surface is smooth. Intake manifold had .001" warp over 6 cylinder run. New exhaust manifold had 0 warp over 6 cylinder run. New manifold gasket is Mopar part. Would assume the .001" warp in the intake manifold would be taken up by the gasket. I have sprayed all vacuum connections with carb clean and verified there are no vacuum leaks at any hoses.

Motor literally has 1 hour of run time on it. I doubt anything has warped significantly enough to cause the "lean" condition that is described above.

Could possibly be an injector concern but if that were the case and assuming it is a rich condition I would think I would see a fuel pressure bleed down issue at both key on and when shutting the key off. Injector pulse width at idle is 4.8 milli-seconds which is about average.

With the computer control on this engine you cannot adjust the distributor to change the timing. The computer will re-correct it. It is possible that the distributor was put in one tooth off, and as a result the cap timing will be off, though the spark timing remains all right. However, that is pretty easy to diagnose and correct if you have the instructions for replacing the distro. It might be a good idea to check that. But if it is far enough off for the engine not to correct, it probably will not run at all.

In my experience with an older 4.0, if the distro is put in one tooth late, it will start beautifully and not run well if at all, and if it is put in one tooth early it will start hard, with the appearance of hydro-lock at times, but will run fine.

I had an old Ford truck that did the same as yours with a remanufactured engine, no matter what I did, and the best guess I could come up with was that the valve timing itself was off. I sold it before I bothered to fix it. It ran great, but the manifold glowed red, and every once in a while the muffler would explode after it was turned off. I wonder here if the rebuilder got the chain a tooth off.

I think at this point it might be time to get the front end open and check the marks on the timing sprockets.

edit to add: i should add that most of what I've read about this suggests that if timing is at all an issue, it will be caused by late timing, not early. Since this is not the issue here, I doubt it is ignition timing. The other thing you might want to check is whether you have a dribbling injector that is pouring fuel. Those can be tested with electronic devices of one sort or another.