1990 740gl running problem

vulcan66 · Post by **vulcan66** » 03 Mar 2013, 20:35

My 1990 740gl starts and idles fine, but when you try to add some throttle, it surges up and down in RPM and it will not accelerate. If you back out of the throttle it returns to a normal idle.

Things I've done.

Cleaned TB
Cleaned IAC valve
Replaced TPS
Replaced MAF
New fuel pump relay
Check fuel volume from regulator
In tank fuel pump replaced several years back
Verified main fuel pump is operational, but fuel rail has no pressure tap and have not been able to verify fuel pressure reading

I read about swapping the radio suppression relay to verify operation. When I swapped the relay's, the car would not start or run at all. It seems that this points to a bad RSR, but it's confusing to me that the car runs and idles fine before the swap and then does not run at all after the swapping of the relays.

Am I on the right track with replacing the RSR relay or could this be a bad rpm sensor or power stage?

Could really use some help.

Thanks

lummert · Post by **lummert** » 03 Mar 2013, 21:16

Does this car have a turbo? If so, check the hose between the turbo and the intercooler.

vulcan66 · Post by **vulcan66** » 03 Mar 2013, 21:18

no turbo

Post by **billofdurham** » 04 Mar 2013, 11:55

I read about swapping the radio suppression relay to verify operation. When I swapped the relay's, the car would not start or run at all. It seems that this points to a bad RSR, but it's confusing to me that the car runs and idles fine before the swap and then does not run at all after the swapping of the relays.

The other relay would appear to be faulty, not the radio suppression relay. I am assuming you swapped it with the radiator fan relay. If so, you need to replace that one to avoid overheating when we get you up and running.

Have you checked for fault codes?

When idling does the tach needle fluctuate?

Have you checked the earth (ground) leads for good connectivity? The main lead from the battery to the chassis needs a good clean contact at the chassis. Also check the lead from the intake manifold to the bulkhead (firewall).

Bill.

vulcan66 · Post by **vulcan66** » 04 Mar 2013, 20:56

I did initially check fault codes that lead me towards a faulty TPS or IAC. Replaced the TPS and cleaned the IAC and thottle body after reading codes.

No, the idle does not fluctuate at all. It idles very smooth and starts fine.

Since the fan relay is clearly bad, could it be that the injector relay is marginal and allowing adequate injector pulse at low RPM's, but with any throttle increase it starts failing to provide sufficient pulse due to cracked or bad solder joints. I don't know if this is a go/no go component or if it can give symptoms like I have when failing?

vulcan66 · Post by **vulcan66** » 04 Mar 2013, 20:57

I will check those ground contacts you mentioned

vulcan66 · Post by **vulcan66** » 04 Mar 2013, 21:10

This may be important regarding my problem in this thread.

Shortly before the previously described problem started with my 740, I had just taken the car in to have the AC recharged.

Car seemed to run fine after the A/C recharge. Then I took it on a short 30 minute hop and the problem with the surging started and there was little to no power and press down on the gas pedal only made things worse. When this happened, I immediately pulled over and opened the hood to find A/C refrigerant leaking from the big nut fitting on what I believe is the A/C evaporator canister.

So we have discovered that the cooling fan relay is bad so maybe that caused the A/C system to go over pressure or something causing the refrigerant leak? I don't know, but it is suspect to me that this acceleration/surging problem presented itself at the same time these other events occurred.

Thanks to any interested in helping