MBMB

1989 928GT.

Idles fine. 776 RPM.

Drives great. But then, after a day of driving, while cruising along ~70 mph in 5th, the mix gets increasingly rich and power drops until there's nothing there. (Meanwhile, fuel consumption rises markedly and steadily.) Engine flooded; oil smells of gasoline.

Let the car cool down and the plugs dry out for about five minutes, and she's good for another 20-30 miles.

I thought I had the problem whipped, but it came back today. Before I put another sumpful of expensive oil in, I want to know that I've fixed it.

I'm thinking LH ECU or MAF, but I can't get my mind around how the MAF would give a bad (high-voltage) signal only after a day of driving.

Where to start?

Thanks,
Mark.

ViribusUnits

Check the fuel rail pressure.

If the fuel return is cloged, or partly cloged, it could cause the problems you say.

I can't imagion why it would be, but just to be safe.

Scott M.

Mark;

Pull a vacuum line (preferably to the damper) and smell for gas.

MBMB

VU - without a pressure gauge connected, I'm not sure how to check the fuel rail pressure.

Scott -- no gas in vacuum line to damper. I'm guessing you were thinking damper was leaking fuel into the vacuum system, and fuel was being drawn into the intake manifold.

The '89 GT's fuel consumption gauge uses the number of injector pulses as the input to figure the gas conspumption. So the falling gas mileage, along with the overrichness, suggests that the injectors are pulsing too often for the amount of air that's being drawn in. If the air supply isn't constricted, the injectors are firing too often.

Why would the injectors fire too often? 1) MAF problem; or 2) LH probleml 3)?

ViribusUnits

There isn't any way to check the fuel rail pressure, w/o a guage.

And your right about the fuel consuption. Too high of fuel pressure, or a leakly regulator wouldn't have any effect of the fuel consuption guage.

Try the MAF first. There is a guy that can check them, and rebuild them on rennlist. It could have a peice of circurty that is almost broken, but connects when it is cold. As the MAF gets heat soaked from the engine, the circurtry get hot, and that could cause your problem.

And don't laugh, I had something happen similer to my ignition control unit. As it got hot in the engine bay, the unit would short out. But it took it a while to get heat soaked, and up to temp.

Good luck.

Rich9928p

Running too rich and high fuel consumption is a common failure mode of the LH injection module. Failing LHs can be intermittent or can fail all at once.

A bad MAF can also cause fuel metering problems.

The easiest way to find out which one is bad to plug in a known good LH module. If the problem goes away, it is the LH. If it doesn't, it is likely to be the MAF.

John Speake

Early MAFs had a thermal fault that can cause the problem you mention. I see quite a few like that- and repair them !

Regards

MBMB

Props to John Speake and to Jim from 928intl. With their help, I decided that the MAF was the most likely culprit. I replaced my MAF with a new unit from 928intl, and I hereby proclaim the problem solved! Bwaaahooooo!!!!!