MBMB

Is there a mode of catalytic converter failure in which the converters cause no problems when cool, but when hot restrict the exhaust so that combustion is incomplete and the car runs progressively richer?

DG84S

Why do you think it's running too rich? Is your HC reading high? Is your oxygen sensor connected and functioning properly? We need more info to help you.

MBMB

After running warmed up for a long while (a shorter while in heavy traffic or high ambient temps), the fuel economy drops as it loses power, eventually stalling out, flooded. It's dumping fuel into the cats. I smell gasoline in the exhaust. If I disconnect the fuel pump and run the starter (to pump extra gas out) I can start it up again. The longer it sits (cooling down) after failure, the longer it will run. At one point it expelled soot from the tailpipe on startup after one of these failures.

If it were running any richer I'd be leaving a trail of premium unleaded behind me like Hansel & Gretel's breadcrumbs.

My question at this point is not "what's causing the problem?" (we've been through that elsewhere, and I have several hypotheses -- bad MAF, bad ECU, bad Temp II sensor, etc. -- to test when I get my current round of repairs done and my ECU test box built) but "do I need to add 'bad cats' to my list of hypotheses to test?"

Pizza

Yes. Add it to your list. You can do a quick vacuum test to determine if the cats a clogging. Do a search on this forum for details on the test.

ViribusUnits

Add a clogged cat to your list of issues, but don't add a cloged cat to the list of things causeing your car to be rich. It probably is, and you car will probably take off like a bat out of he** with it fixed.

The LH-jet EFI injectes the neccary amount of fuel for the quantity of air that goes through the MAF. A cloged cat reduces the air going through the engine, and thus the air going through the MAF, resulting in a reduction in the quantity of fuel injected, if that make sence.

MBMB

Pizza -- no luck finding cat vacuum test procedures.

VU -- the car goes like a bat out of hell now . . . until something gets hot and fails and it starts riching out. My thinking was that if the cats were clogged, the exhaust wouldn't be getting out, would be backing up into the cylinders and stifling combustion. If combustion were stifled, the same amount of air could be sucked into the cylinders through the MAF even while less gas exited, so the ECU would keep dumping fuel into the engine, which would look a lot like the symptoms I'm getting.

I'm rebuilding oil cooler hoses and PS pump hoses right now, so it'll be a few days before I'm back on the road and can replicate the problem.

John Speake

What you describe is a well known MAF failure mode. Another of Mr. Bosch's design problems.

My money's on MAF failure.

Jim bailey - 928 International

Nope .......but continuing to drive it WILL probably destroy the cats melt the ceramic and plug the exhaust permanently . Sometimes the ceramic honeycombs break and bounce around when it rotates so the small holes no longer point through the cat it can block the exhaust flow but then they rattle . Your car has basically two cats each with three ceramic combs even if one were blocked the other is good for at least 175 hp . You have a fuel problem and most likely mass air sensor ..............

MBMB

John,

I replaced the MAF (with a new one from 928 international) and the problem persisted.

So now I have either two functional MAFs, or two MAFs with the same failure.

Any suggestions?

Randy V

My initial suspicion is that a coil is failing upon heat loading, resulting in incomplete ignition and raw fuel dumping out the exhaust.

John, having seen every type of MAF failure, most likely has the actual diagnosis.

MBMB

Randy, my 89 GT has the ignition monitoring system. If this were working properly and a coil failed, the fuel injectors on the cylinders served by that coil would shut down, which would result in a loss of power but no raw fuel being dumped.

Jim, I caught your reply after I sent my last. Any suggestions for testing the new MAF I got from you to check whether it is in fact bad?

Randy V

Ah yes, I forgot that system was implemented on '89 onward.

DG84S

MBMB,
A cold engine requires a richer mixture, that's why yours seems to run better the longer you let it cool down. Once it warms and needs a leaner mixture, it doesn't get it and it drowns in excess fuel.
I would start with the circuits that control the length of time your injectors fire (fuel pressure is constant, so amount of fuel is determined by how long the injector is energized). I think your problem lies in the MAF, ECU, or Temp II sensor. Next I would look at the amount of fuel that each pulse releases. Too lean and I would suspect clogged injectors or low pressure. To rich and I would look for leaking injectors or failed fuel pressure regulator.
Your cats may be damaged from the contamination of raw fuel and ultimately need replacing, but I believe the cause of your problem is upstream in the circuits that tell your engine how much gas to drink.
Good luck...keep us advised as you work through it.

MBMB

DG --

It's something more than being "warmed up" that triggers the problem. It'll run just fine when cold, and then when the coolant is hot, but after running that way for, say, 90 minutes (shorter if it's hot out or I'm running in heavy traffic) it starts getting gradually richer. So for that period everything is working fine. Something is getting heat-soaked and failing. Since the ECU is not subject to engine heat, I guessed that the MAF was failing. The persistence of the problem with a new MAF led to second thoughts.

I'm going to build a box to check the inputs to the ECU on-the-fly. Parts are on the way.

Jim bailey - 928 International

Mark I wonder if someone has hotwired the Ignition monitoring relay that controls the injection to two cylinders on each bank ?? the 89 is known for false shut downs and often is "rodent engineered" . How about unhooking one of the plugs on the ignition secondary stage and force it onto 4 cylinder mode and check to see if that funny clear relay turns red or green ?and if the power and emissions are similar to the other event which happens after an hour or so of driving ..