Exhaust Troubleshooting

Last updated: May 9, 2000

Date: Mon, 8 May 2000 12:35:32 -0700
From: "Ulen, Robert S" (Robert.Ulen@PSS.Boeing.com)
Subject: (rx7) [3] Exhaust Leak Findings - Lessons Learned

I though I would share my trouble-shooting results for those of you may one day detect what sounds like a slight exhaust leak on your FD. This write-up could save you some time, and point you in the right direction.

But first, I must pass on my axiom which applies to trouble-shooting this car sometimes:

- ------------------------------------
"Complex problems have simple, easy
to understand wrong answers."
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Based on the responses I got from the List, this problem seems very rare, and the symptoms may vary depending on how bad the failure is.

A LITTLE BACKGROUND

After driving hard one day, I popped the hood in my garage (quiet background) while the engine was running, and instantly notice what sounded like a slight exhaust leak near the turbos. After studying up on all the possible things that could cause an exhaust leak, and some advice from the List (thanks to all who gave me some advice), I started a long, and careful trouble-shooting plan which paid off in the long run.

POSSIBLE CAUSES

I determined that there are many possible sources for an exhaust leak on an FD, namely:

1. Any gasket associated with the exhaust manifold and turbos or downpipe. This means removing turbos to fix.
   
   
2. The lower intake manifold gasket where the EGR gas port for the airpump system port air transfers between LIM and the rotor housings. This mean removing turbos, extension manifold and LIM (worst case problem). Note: my car is a 95, and it also has the paper lower intake manifold gasket like the 93/94 FDs, which is very prone to leaking where the exhaust port air passage is located.
   
   
3. A bad port air check valve (in airpump system air control valve) or bad check valve in the spit-air tube between intake and main cat. This means removing extension manifold for port air check valve, or just replacing split-air check valve (easiest fix - so you know its not that).

POSSIBLE SOLUTIONS

The following match with the numbers above:

1. The only thing you can do is remove the heat shields from the downpipe and turbos to inspect for any loose or missing bolts/nuts. You can also perform a "cold start" test to look and feel for any obvious leaks. Not all areas can be seem or felt, so it depends where the leak is if you luck out or not.
   
   
2. If you remove the shield underneath the tranny, you can actually see the very back, lower area of the LIM where the EGR transfer port is. You have to look up, and between the pre-cat and the engine with a flashlight to see it. I saw some oily residue at this area, and though for sure this was the source of the leak. A cold start test relieved no noticeable leakage. You can actually get a finger up there to feel for a leak, but only for about 30 sec until the pre-cat gets too hot. The oily residue could be from something else - don't know for sure.
   
   
3. If you remove the hose between the airpump and the metal tube going to the air control valve (its supply line) there should be no soot residue inside it. If the engine is started, and any exhaust gas blow out of the is hose, the port air check valve is bad. This is what was wrong with my car. Apparently, the exhaust gas was blowing backwards through this check valve into the lower intake manifold near the turbos. It sounded just like a basic exhaust leak. What was strange when I listened to the leak noise, is that it would seem to disappear with some rpms. What was happening, is the airpump would overcome the exhaust, and the sound would stop. I think this check valve has been bad for awhile, and it finally "let go".

So, it means I must remove the extension manifold to repair this. No biggie, it will give me an excuse to do the silicone hose/tie wrap job too.

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From: Steve Cirian (steve@ScuderiaCiriani.com)

My car was making a hissing noise that sounded like it was coming from the exhaust in roughly the transmission area. It turned out to be a bad gasket for the airpump hose going into the main cat. There is also a short piece of rubber hose in that area that could split or come off (not that poppoing off is very likely given the clamps on it).

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The pre-cat can become clogged. This shows up as poor idle and/or boost problems, and the pre-cat will sometimes even glow red after extended idle. (Note that it is common for the pre-cat and even downpipe to glow after hard driving.) --Steve

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Date: Tue, 30 Jan 2001 23:34:53 EST
From: Pomajavich@aol.com

>exhaust overheat light is on

The exhaust system overheat indicator is a simple sensor that is a thermister. That is to say that it is a resistor that changes its resistive value as it is heated or cooled. The circuit is an op-amp voltage comparator that has a window value built in that will activate when the thermistor reaches a certain value based on heat. The circuit must sense some resistive value all the time or it will activate.

It is reasonable to assume that somehow your sensor(thermistor) has come disconnected from it's cable going to the comparator portion of the circuit. The sensor is under the carpet over the catalytic converter. It requires a bit of work to get to. However there are places where various harnesses plug into each other more accessible.

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If you are looking for exhaust smoke information/troubleshooting, see the engine smoking troubleshooting page.