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'86 740 Turbo Sedan idles very roughly at stop lights

830 views 2 replies 2 participants last post by  OrangeAirsoft 
#1 ·
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?
 
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#2 ·
Re: '86 740 Turbo Sedan idles very roughly at stop lights (OrangeAirsoft)

Too much LA air - both car and driver?

You car may have a vacuum problem - both engine and brake related.

Check ALL vacuum lines, fittings, connections, valves and controls that need vacuum to operate. This means under the dash and through the firewall also. Make sure the brake system has perfect vacuum and controls. While you are stuck up-side-down under the dash check all electrical grounds and connections. Does the turbo have vacuum lines?

My guess is a poor connection of a single vacuum line.

George Dill
 
#3 ·
Re: '86 740 Turbo Sedan idles very roughly at stop lights (RearWheelPaul)

Thank you very much for your replies, I will try check them myself, and consult my mechanic. I just needed to know as much as I can before I went to the mechanic. They might sense I don't know anything (which is actually true
) and might charge me an arm and a leg. If I gave them some input, they might think I know enough to prevent them from overcharging.

Oh and if anybody knows a good Volvo mechanic in the LA area, please let me know.

"too much LA air" : well, yeah... this car was in Seattle, Wa for 19 years, never had any problems. Three months in LA, and it starts to have problems. So could be the air.
 
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