'86 740 Turbo Sedan idles very roughly at stop lights.

Post by **OrangeAirsoft** » 27 Jun 2005, 01:04

I know there are plenty of topics covering stalling and rough idling 740's all over this board. But mine has something specific that might help the Volvo experts here diagnose what exactly needs replacing on this specific vehicle.

My 740 has 185k miles, and runs great when cruising at high speeds. No engine lights are on. No loss of power at freeways, runs smoothly. At the stop light however, 8 out 10 times, the engine will start to idle wildy, the tach dropping to 300rpm for a quarter second and going back up to about 800rpm in a seesaw fashion making the car sound like its about to stall. It doesn't stall however, the tach just goes up and down(but not to 0). At one point, it stalled when I released the gas too quickly in the parking going very slow (about 5mph). But seldom does that though. Sometimes, it would stall in the street too, releasing the gas too quickly, but starts right back, and the car goes like nothing happened. The tach doesn't really go zero on me its just jjumping up and down wildy when I'm on the brakes at the stop light, and I'm on Drive. Putting the car on Park, the car is fine. Neutral also has the seesaw tach problem, but not as bad as when the car is on drive. Turning the A/C off also "lessens" the effect a bit, but it is still there whether A/C is on or off.

NOW, HERE IS THE INTERESTING PART...

Everytime I think "I'll try and fix this when I park it.", the engine would suddenly start working right, and so I couldn't diagnose the problem further. As soon as I'm parked at my driveway, with my foot on the brake and that car still in Drive, the problems goes away all of a sudden and idles smoothly. So, after a lot of cancelled "I will check this when I get off" plans, I realized that while running on the street and stopping at red lights, I have the problem. The moment I pull into a parking spot and need to use the reverse to park, the problem goes away like nothing happened. All this while keeping my foot on the brake, and the car is still in Drive. Does it make sense? Using Reverse for a few feet fixes it? Too weird for me.

Volvo gurus, are you able to diagnose this problem?

M Jarve · Post by **M Jarve** » 27 Jun 2005, 08:19

I could possibly be the hall sensor near the distributor. This somtimes fails, and stops sending RPM data. I think the computer then begins to stop the engine, thinking the timing belt broke, and preventing engine damage.

I believe the one on my 940 is starting to go too, as once in a great while (maybe one time every 6 weeks) I'll be stopped and my engine will start the same thing.