Hey all. My '91 740 sedan will not idle correctly. I tried starting it todayand it will idle at about 100 rpm for about 1 second and will then die. I can get it up to about 1000 rpm by putting my foot on the gas, but any lower than that it will feel like it's skipping a cylinder. But here's the thing, it will ony do that on a warm start. Starting it today from a cold start it stated just fine until it warmed up. The tact then started to climb and fall between 1000 rpm and 250 rpm for about 3 seconds until it died. I checked the fuel lines and the fuel pumps and they are fine. Could this problem be something along the lines of the vacuum tubes? What could also cause these problems? I tend to fill up on 87 octane fuel when the owners manual calls for at least 91 octane fuel, so could that also be one of the problems?

 

leaky vacuum lines wouldn't help, but make sure the idle switch on the throttle body is working (should 'click' when you take the throttle off idle, and you can easily test it with an ohm meter to make sure its switching properly). when the idle switch is closed, the idle control valve is modulated to keep the engine at its optimal 700 rpm idle. when its cold, they keep the idle a little higher by richening the mixture.

any faults on the LH2.4 diagnostic module? see Engine and OBD Diagnostic Codes for how to read these codes (no tools required at all) and what they mean.

yes, a turbo does run better with premium gas, but that won't affect idle, just performance under high throttle (with lousy gas, if/when it knocks, the knock sensor causes the ignition to retard, reducing overall performance and gas mileage)

 

Just one smal problem, I don't have a code reader for the car.

 

Use the link that Pierce supplied. No code reader required

 

Wait, nevermind I found it. Going to test it now.

 

OK, got the code 2-3-2. That means that the fuel trim is either too lean or too rich at idle.
So afterwords I started the car and let it warm up. It warmed up to operating temperature with no problem. I then decided to drive it around the 0.5 mile circle that I live on. It only seems to die when I drive it. Could that be the high pressure pump is not working properly?

 

2-3-2 is a pretty generic code, can be caused by lots of things, from a weak O2 sensor to to the MAF/AMM (mass air meter) to air leaks in the intake or exhaust paths.

but... your symptoms seem to be changing, before, it wouldn't idle. now, its stalling when you drive it.

a failing high pressure pump will generally cause total no-start problems as it won't be delivering any fuel pressure... or a clogged fuel filter and weak pump will run fine until you step on it at medium to higher RPMs then the engine will starve for fuel but recover as soon as you back off. a failing in-tank fuel pump would cause power loss when the gas tank is less than half full but drive mostly OK with a full tank, although it would cause your main pump to strain harder (audibly so).

 

OK, just foun out that the oxygen sensor on exaust could be the problem. The car being 21 years old and burning oil tends to gum up the sensor. If that is not the case, then I'll come back.

 

I believe that this is the oxygen sensor seeing that there are no more wires going anywhere else on the exaust lines. My only problem now is how to get the sensor off. It is stuck there very well. I think that it plugs in and you would be able to get a deep socket on there to take it off.

 

The truth is your problem could be a number of different things. I love when people say hey, my cars doing X what is it exactly as if there's one clear answer for every problem. automobiles are not that an exact science by any means....

Volvo Maintenance Hints for 700 900 S90 V90 Models
If your serious about learning this stuff and figuring out your problem I've found this link every helpful. also the 240 bently manual has a ton of really good info.

 

yup, thats the O2 sensor...

squirt some Kroil around the nut to header interface, let it soak in, and I've always been able to get them of with an open end wrench. use some copper antiseize on the new one being VERY careful to only put it on the threads and not touch the dome shaped tip.

 

I'm new to all this, but my first thought was the pump in the tank. My guess is the pump is fine, but the short piece of neoprine hose that connects it to the fuel system is cracked and the functional pump is just pumping back into the tank and not enough going to the high pressure pump.

 

I finally got the sensor off. It takes a special socket to get on the sensor and then all you need to do is twist. Kroil helps a lot when you let it soak over night. Ordered a new sensor as well. Really hope that this fixes this problem.

 

I have had something like that happen to me at one point. Instead the hose came off the line and the high pressure pump would only get about half of the gas in the tank untill I either top off or fix the problem. Eventually fixed it, but that was actually my first guess at what was wrong.

 

running the car with a defective in-tank pump will really stress the main pump, too, and could shorten its life. when my intank pump crapped out, my main pump was very noisy, even with a full tank. also my power was down as the main pump couldn't deliver enough gas for full throttle at higher RPMs.

 

Good news, the car runs again. Meant to post this a while ago, but it works. The only thing that i think that is wrong with the car now is that it feels sluggish upon acceleration in third gear.

 

Well, what fixed it?

 

What fixed it was replacing the oxygen sensor.
Here's a pic of the old one.