91 q bad ecu?

[Fridge]

I have been working on this car on and off for a while. I work way toomuch, and it is dark when I get home. It has had a intermitent problem. The car runs great then when I stop at a store or something and jump back in its dead. No fuel pressure. Then after a while it would start back up and run fine for a few hundred miles. I replaced the fuel pump. Did it again, I replaced the fpcu, ran great for a while then completly died. Fuel filter is new. I wired around the fuel pump relay and it started right up with 50 psi. As I was warming it up she died again. This time I still have fuel pressure. Sigh

Any help would be deeply appreciated.

[Q45tech]

As with MAF, CAS, ECU the fastest way is to swap a KNOWN GOOD UNIT to diagnose then repair the defect at your leisure.

Intermitents are usually solder or connector related.

[Fridge]

Well,Its not the ecu. I bought one from the local salvage yard and it does the same thing. The car will run fine untill it starts to warm up and then it dies. I dont think its getting power to the injectors since I am still getting good fuel pressure. If I let it cool off for a while it will start right up.

Where do I go from here?

[Fridge]

OMG have I stumped the nico techs? I have a fsm but it is the little one and hard to read. I have to use a magnafying glass to see the tiny print. So I will try to locate whatever controls are affected by the heat sensors. My cash is getting low so I cant keep just replacing parts. I sure miss this car she looks so lonely sitting out in the yard

[Q45tech]

One normally uses a Consult to datalog all the parameters then spends some time observing the 20 or so numbers to zero in on the culprit.

A real qualified technican should be able to visualize [every one of] the correct sensor output voltages/readings.

This technican will have worked on Q everyday for the past 10-17 years.

INET diagnosis is usually just a regurgitation of common failure points, not being there is difficult at best.

Common for defective solder joints to become sensitive to heat as they warm up from having current flows they change resistance or open totally.

Common in MAF* and FPCU..................less common in CAS and ignitor switching module but occurs.

MAF is the most common electronic failure item in the engine bay!

[Fridge]

I was just playing you Q, I respect your knowledge and this forum is really the only reason that my q is still on the road. So does the computor look at the maf after a certain lenght of time? I mean you can almost set your watch by the time it will run and then die. I think I saw a post where someone bypassed the maf for a limp home mode but I can not find it. I got home early today I will see what I can find.Thanks

[maxnix]

I thought the ECU monitored the MAF continuously.

How about your ignition switch? Fuel pump relay?

[Fridge]

Okay, I think I got it. It was combination of the ecu and the maf wiring. I pulled the connector from the maf after the car died and cleaned it and put bulb grease on the termnals and she fired right up. I put the fuel pump relay back in and it still ran. I put the old ecu back in and no fuel pump. So I put the salvage yard ecu back in and so far 100 miles with no problems.

2 problems at once had me confused but she had been sitting for a while. So I guess I`m in the market for a new maf and connector.Thanks Q you pointed me in the right direction.

[Q45tech]

IF the MAF appear ok for extended periods then fails, then works again you probably have a intermittent solder connection where the connector terminals meet the circuit board or where the sensor terminals connect to the circuit board.

Transistor and IC don't work intermitently, when they fail, they fail forever.