MBMB

When I have been running for a while (about an hour; shorter at higher ambient temps or lower speeds; shortest in hot weather and traffic), I start running rich to the point of stalling. If I then let the car cool down for 20 minutes or so, I can start up again and resume running for a comparable period.

My thinking is that something is failing when it gets heat-soaked. This led me to believe that it was under the hood. I replaced the MAF (new one from 928intl), replaced the coils (new), and replaced the ignition wires (new). No improvement.

The LH ECU seems to work fine in a friend's GT. The failure mode seems inconsistent with ECU failure. PO had replaced the computer in '98.

No fuel in the vacuum lines to the FPR or dampers.

Fairly (~1500 miles) fresh fuel filter.

When it is running poorly, it is not running one one set of cylinders -- disconnecting either ignition final stage has the same effect.

I tried disconnecting the MAF when it was running poorly; it wouldn't run at all. So I think the MAF isn't the problem.

I think the PO sold the car with the problem. Knock sensors were replaced in '02 for a condition described as "engine cutting out."

Any ideas are welcome. I'd like to get her running right in the next 10 days so I can drive to Wichita.

I may borrow / rent an LH ECU to rule that out once and for all.

I'm thinking about installing a fuel pressure gauge to see if the problem is related to fuel pressure.

Chris

Have you checked the relay down by the ECU to see if it is making the engine run in 4 cylinder mode ? Maybe one of the sensors on the head is bad.

Just a thought.

Chris

Normy

T2 sensor? Check the resistance per the manuals....

N!

worf928

What are the components in the system that can make the car run rich?

- O2 Sensor
- Computer
- MAS
- Temp II sensor
- Fuel Pressure

Anything else?

You do not mention having replaced the Temp II sensor - at least not in this thread. That sensor tells the LH whether or not to enrich for a cold start/cold running. Have you checked the resistance of the Temp II sensor on both circuits when cold? When hot? When headsoaked? I've heard that a toasted Temp II will richen the car to the point of belching black smoke - never seen that myself though.

Quote:

I'm thinking about installing a fuel pressure gauge to see if the problem is related to fuel pressure.

That is good thing to have regardless.

You are thinking that you have too much pressure once the motor gets heatsoaked? I would think the reverse would be more likely - low pressure when hot. Hmmm... do you have a stock FPR? Or maybe someone snuck an adjustable in?

Garth S

"When it is running poorly, it is not running one one set of cylinders -- disconnecting either ignition final stage has the same effect."
I'm not exactly sure what you are saying: ie., is it only firing on 4 cyls. when it is running poorly? If so, do you have an extra ignition control module to swap in (re the 2 near the rad. )
The '89 has the red/green LED relay on the EZ-K that shows whether Ign circuit I or II is in trouble - does either of these LEDs come on at any time? (If either ign circuit cuts out, the thermal sensors in the exhaust manifolds [cyls 3 & 7 - or 4 & 8] are there to indirectly shutdown the injectors for that particular circuit - to avoid a too rich condition): Perhaps that safety interlock is not functional which would allow a rich condition if there was an ignition problem as mentioned above.
The recent revisit of the '87S4 struggle Jim of Euroworks had with a rich condition had involved IIRC, exchange of an LH unit that was also faulty - which unfortunately hid the problem (as due to the LH).

MBMB

Chris, Garth: it is not running in 4-cylinder mode. The ignition control circuit is functional (tested by running with one ignition circuit disconnected). Sorry for the lack of clarity.

Normy, Dave: I haven't replaced or thoroughly tested the T2, but the problem occurs with the T2 disconnected as well. I have a stock FPR, as far as I can tell.

The other thing that could make the car run rich is a stuck injector, but I'm not sure how a stuck injector is consistent with the problem arising after running for a while.

John Speake

Mark,
If you disconnect Temp sensor 2, the car will think the engine is very cold, and run at max rich mixture. The test you did is not valid.

Does the car stay running rich for some time when it gets hot ? If so, this would give you time to check T2 resistances etc.

Can you get the car to someone with a "Hammer" ? - that could help to find a sensor issue.

MBMB

The car will stay running rich for long enough to check T2 resistances.

I put a hammer on it, and the only issue was with the idle control valve not responding.

John Speake

When you swapped your LH into your friend's car, did you run it long enough to get the same (heat related ?) problem ?

Have you monitored O2 sensor volts when running OK, and when running badly ?

MBMB

1. I thought I ran it long enough, but possibly not. I'd like to find a loaner LH to put in my car to rule out ECU problems.
2. No, but I'm planning to do that on my next run (along with checking TII readings).

John Speake

Some components in the LH do run quite warm, but I don't think any LH failure would "reset" itself after it failed.

You may find either temp sensor 2 or the O2 sensors are failing when hot.

Have you run a diagnostic tester when it was in the failed state ?

MBMB

I hooked up the hammer when it was just starting to rich-out.

Would TII or O2 failures cause such a dramatic overrichness that the engine floods?

John Speake

The effect that open circuit sensors give depends on the MY software used, but for your car, if there are no other faults, then the software will use default values, which are typically in the mid range for a engine at normal temperature.

I've just been running the GT software on my test jig to confirm this.

If the Hammer is not reading out a currently present fault (when failed hot) or reading a previously stored (intermittant) fault, then this may suggest a faulty LH ECU.

The LH should have at least registered a fault that the O2 sensor reading was out of range when read with the Hammer..

Jim's faulty LH ECU had not only an O2 loop problem, but also it "thought" that "throttle closed" and "WOT" switches were both closed, when I checked it out here.

I have an ECU you can borrow with GT software, but you may find one closer to home.

I agree that it "sounds" like an engine bay component getting heat soked. The prime suspect there would be the MAF. They have a design fault where they experience a thermal runaway fault, which shows itself with exactly the symptoms you describe. But I know you have already changed that.

Regards,

worf928

What about wiring harness continuity to the MAS? Crufty contacts at the harness connection to the MAS perhaps?

MBMB

Dave,
Contacts at the harness-MAF connection look good. When it's running poorly, if I disconnect the MAF it won't run at all.

John,
What would it cost / how long would it take to borrow your LH ECU?