I have a 92 Volvo 740 with 396K miles.
When I start the car in the morning, the car starts to idle for 30 seconds perfectly, but then it starts to go up and down in the rpms. Until it warms up it's fairly quiet. But if I turn it off and wait an hour and start it again, the idling goes up and down, until it gets a bit warmer.
I just recently replaced the O2 sensor next to the catalytic converter. I've replaced a lot of sensors and relays. I've cleaned the throttle body. Replaced the distributor and cables. (I've read that they need to be "Bougicord". if they're not, when it rains, it idles extremely rough until they dry up. I'm in the process of changing them, though.)
Still, the idling part have me at a loss. Any ideas?

 

Hi, even though i'm a relatively new member here,here is the obligatory question. Is your car a Bosch or Bendix Regina/Rex fuel system. If you don't know, you can tell by which side of the car is the Ignition coil located. Big square coil on drivers side it's a Regina Rex. orginary looking cylindrical coil on the passenger side it's a bosch. "i'm assuming that this is a US/Canada market car".

 

It is Regina. Square box driver side.

 

OK i thought so, i have a 89 volvo 740 wagon with the same system. I believe 89 is the first year for regina rex system and has some differences from the later years 740/940 regina cars.The good news is regina rex is a great system that ran for over 20 years without any/many problems. The bad news is after 21 years there are possibly several things that contribute to bad idleing. 1st is hidden vacuum leaks, in my car the vacuum line going from the manifold to the MAP sensor is made of several parts; rubber piece from manifold to yellow spagetti looking hard line and another rubber piece going from spagetti line to map sensor if you can slip these parts from one another without much effort it is a possible leak area. also check around the fuel injector o-ring seals on the intake manifold. mine were original to the year build of my car and were loose and leaky. you said that you have cleaned the throttle body and it is spic and span and used a new tb gasket? 2nd if you have experienced power drop outs at slightly above idle throttle positions i would replace your map sensor, "mine was also made the same year as the car was made" "it acted alot peppier after i replaced that". 3rd it could be a worn/dirty IAC valve or leaky iac hose, these are made by VDO and are expensive new and from what i have read you want a new one. although i found on a different forum that someone retrofitted a worn out Bosch IAC valve "weaker internal spring strenght, closer to stock VDO spring" as a substitution and it performed excellently. "I haven't performed this yet and don't recommend this until further testing has been performed" YMMV

 

PS you said you replaced alot of sensors, did you already replace the map sensor? if so did you buy OEM , cheap aftermarket GM equivalent or better grade aftermarket "lifetime warrantee" version? I got a real cheap one and it idle hunted out of the box, upgraded to the premium version and it got alot more civilized.

 

FYI my 89 740 GL is a Bosch system . It also does the same at cold start but after less then a minute it goes to normal idle. I don't consider it to be a issue to spend time on. The car only has 353K miles on it and just got it's 350K tune.

 

Mr. Newbie:
Yes. I cleaned the throttle body and got it a new gasket. Replaced the map sensor. Replaced the oil trap (to which, to this date I'm unsure what is it that it do) and flame trap and hoses. Replaced the gasket on the intake manifold. Replaced the injector that goes into the intake manifold (I don't remember the name, but it is the blue expensive one). Replaced the temp sensor (the one with the square, thin prongs, not the one with large, round prongs). Replaced the knock sensor. Replaced a couple of lines where a vacuum leak was evident (you could hear it). Haven't looked that closely to the injectors, but when i cleaned the intake manifold they seem fine. Replaced the IAC valve form one in the junkyard. But it behaved as the one previously installed, so either I really have bad luck or it is in acceptable condition. I cleaned it thoroughly before installing it with brake cleaner (which is the cleaner that I found most efficient). And no, the retrofitting part sounds weird, so 'no' on that one. Replaced the O2 sensor. And yesterday, re-soldered the exhaust pipe. Got broken due to an oil leak in the oil pan gasket which make the engine move more than normal. It's in the "to-do" list with the motor and transmission mounts.
And Mr. Jagtoes:
Mine does something similar. When I start the car, it idles nicely for about 30 seconds, then goes up and down (and on the meantime, I'm unable to move it, loses power) until it gets warm (above the line of the temperature gauge), then I can move it. Now, even if it at the middle of the temperature gauge (normal running temperature), when I stop at a light, the idling goes up and down from 600 rpm to 750 rpm. if I put it in neutral, the idling goes from 700 rpm to 950 rpm.
I believe that that ain't normal idling conditions, but then again, I could be wrong.

 

Hi again Mauricohg, Here is a link to the post i found on the turbobricks forum. The original poster is #!rolls2
740 Rex Regina idle win [Archive] - Turbobricks Forums
here is what happened to him using Junkyard VDO IAC valves
"Admittedly, I haven't purchased a new VDO IAC valve. At the same time, I have tried three different (cleaned) used ones. No joy, until this afternoon.

I installed a nasty old "wo out" 2.4 Bosch IAC valve (2 pin) just for yucks and the car idled like a NA Volvo should idle - maybe for the first time in many years - who knows. I drove the car for 35 miles with no codes (apart from the what should have been factory installed 3-2-1) surfacing."


Not saying you have to use a bosch valve but maybe a brand new VDO valve is the ticket.
YMMV

 

Most cars have the Bosch system, it was optional at the time. with my car and also Mauricohg's the idle hunted so much it was in danger of stalling, very unnerving when merging onto a road or accelerating from a green light.

 

note too that any cracks in the rubber hoses to/from the idle valve will make things ugly.

 

Ours is ready to drive a startup. Idle goes to 1000 rpm then drops slightly and runs rough until you put it into reverse and back out of the garage. When you put it in drive the idle is at around 700 rpm and it drives normally. Don't need to warm up the car just start and drive. Does not stall or even gets close to stalling. Motor just shakes a bit then OK.