90- 740GL stumbles at idle

Ron Richison · Post by **Ron Richison** » 09 May 2009, 14:46

Problem ongoing for 2 years now, I'm still working on it and am determined to fix it. This car "stumbles" at idle only. Runs fantastic on the road. It has never caused the car to stall, but it sure feels like it might.
Here's what I've done; new parts:
MAP sensor, air idle control, idle position sensor, oil temp sensor, flame trap, fuel filter, cap, wires, plugs, fuel pressure regulator, both fuel pumps, injector seals, crank shaft position sensor, all vacuum lines.
Here's what I've done other than parts:
Cleaned throttle body, thoroghly checked for vacuum leaks (found none), checked compression(1,2@ 135 and 3,4@ 140).
Idle is set to 800.
Any more ideas? I know replacing parts is not a substitute for proper diagnosis, but I was running out of time and options for my kids car. Now he drives a Jeep and I want to keep this car and make it run right.

Post by **billofdurham** » 09 May 2009, 15:28

Coolant temperature sensor and connector?

This link shows how to locate and check the sensor: https://www.matthewsvolvosite.com/forums ... tor#p81347

If you haven't already done so check for fault codes.

Bill.

Ron Richison · Post by **Ron Richison** » 09 May 2009, 17:33

yes I did change that H2O sensor, sorry I did not list that one. Isn't there also an oil temp or pressure sensor? If so, I have not replaced that.
No codes.

Post by **vegasjetskier** » 09 May 2009, 18:17

Ron Richison wrote:Problem ongoing for 2 years now, I'm still working on it and am determined to fix it. This car "stumbles" at idle only. Runs fantastic on the road. It has never caused the car to stall, but it sure feels like it might.
Here's what I've done; new parts:
MAP sensor, air idle control, idle position sensor, oil temp sensor, flame trap, fuel filter, cap, wires, plugs, fuel pressure regulator, both fuel pumps, injector seals, crank shaft position sensor, all vacuum lines.
Here's what I've done other than parts:
Cleaned throttle body, thoroghly checked for vacuum leaks (found none), checked compression(1,2@ 135 and 3,4@ 140).
Idle is set to 800.
Any more ideas?

I didn't see ignition coil or distributor rotor on your list.

Ron Richison · Post by **Ron Richison** » 09 May 2009, 22:35

rotor is fine. how would ignition coil relate?

Post by **vegasjetskier** » 10 May 2009, 08:54

Ron Richison wrote:rotor is fine. how would ignition coil relate?

Just to be sure that you are getting a good spark.

Ron Richison · Post by **Ron Richison** » 11 May 2009, 09:12

ok, here's my thought: I have good spark, fuel delivery and it's running well mechanically. Could I be getting faulty info being sent to the ecm forcing it to make "adjustments" to correct something that's not really there?
Is there a way to by-pass / analyze signals being sent from all the components that would be responsible for the ecm to make adjustments?

there are no codes being sent under normal operation. I know codes CAN be sent because when I unplug components and restart the car, it will recognize it and send the appropriate code.

Ron

Jaywalker · Post by **Jaywalker** » 12 May 2009, 07:16

PCV valve?

Ron Richison · Post by **Ron Richison** » 12 May 2009, 19:58

on this car it's called a "flame trap" and yes, that was the first part I put on.

anyone care to comment on my ecm theory? false signals that trigger a fluctuation. can I bypass (jump) some of the components that send signals?? no one seems to think that it could be the computer itself, so maybe it's signals coming in??
help.

Tiki · Post by **Tiki** » 12 May 2009, 22:20

Ron Richison wrote: anyone care to comment on my ecm theory?

Can you borrow an ECM from someone and try it?

Some mechanics keep one or two as a spare under their toolbox. If you do test it without installing make sure it's well gronded.