My car (1989 740T wagon) has been hunting for the proper idle for about 5 years now. It will vary from 500RPM to 1200RPM and back in a period of about 7 seconds. Sometimes it’s not so extreme, but the cycle speed seems to be pretty constant. Also, when the car comes to a stop light and the car goes to idle, engine speed will drop and the lights will dim, but then it recovers. It will remain steady for a few seconds and then start hunting.
Problem first occurred when I pulled off the intake manifold to clean the crank case ventilation system and the oil separator. After reassembly the car hunted after warming up. Car does not instantly hunt when starting from a cold engine. Car still performs very well. It just doesn’t want to idle at one set speed.

Tests done to the Volvo:

Extensive intake leak tests, several found and fixed. Pressure testing the intake revealed a small hissing noise. Some of it seems to come from under the valve cover. It is probably air venting around valve stems. As I have plugged the last few leaks, it seems to not hunt as far.

Mass Airflow sensor swapped with a known good one. From what I understand, these either work correctly of don’t work at all.

CO content enriched a little, as it read lean (by LED test), reaction time seemed slow to the adjustments.

Timing checked at 12 degrees BTDC.

Throttle Position Sensor checked and set properly.

New FI engine temperature sensor swapped in.

Knock sensor swapped with several others.

Throttle body cleaned and new gaskets installed, no leaks now.

New cap and rotor made no difference. Old ones looked fine. Plugs looked good. New wires (poor quality, as the connector falls off almost every time one is pulled off, so I am considering getting new ones) no difference.

Fuel Pressure regulator smell test: passed. Pressure readings have not been taken. Can they half fail?

Oxygen sensor (lambda sond) resistance tests: passed. Swapped in another sensor from a car the idled properly.

Crank case ventilation system checked, good air flow. Cleared the small orifice in the elbow above the oil separator box, it was totally plugged (then adjusted CO).

Main fuel pump seems to run fine and is not excessively noisy.

Fuel filter replaced.

In tank pre-pump replaced. Idle stabilized for a week, then went back to hunting. Wierd?

Air Idle Control Valve passed resistance tests and was cleaned. It seems to work, as disconnecting the idle set ground causes the engine to race a bit and then settle back down.

Gas mileage does seem to be one or two MPG lower than a few years ago.

Maybe Oxygen Sensor is out of whack?

Maybe the FPR is giving low fuel pressure? This would seem to give me a lean condition according to the LED test on test point 219. When I compensated for the lean mixture and made it rich, the idle smoothed for a few seconds and then hunted again even with a lean condition showing by the LED. So maybe it’s not.

So I am open for thoughts, feeling, comments of my true idiocy, similar stories to cry about and etc.

 

You likely answered your own question with the first paragraph. Vacuum leak still. It doesn't do it when cold because the enrichening circuit is still running. May I ask how you plugged the leaks?

 

"In tank pre-pump replaced. Idle stabilized for a week, then went back to hunting. Wierd?"

I would re-visit this repair. Were the correct in-tank hoses used? Could something have loosened up? Maybe a fuel pressure test would tell the rest of the story.

 

Sorry about my slowness for an update. I went from final exams to flying home for Christmas all in a big hurry.

Crusian, I like your thoughts on revisiting that fix. I think a pressure test is a good idea. I do hear the pump in the tank, when I listen for it and I reused the same fuel line in there. I looked to be in really good shape, so checking to make sure that I have proper pressure sounds like a good way to see if that could be the problem. I think I had a local volvo specialist check the pressure before I changed the pump and it checked normal, but I'm not sure that I trust that mechanic.

Cycleboy, I think this problem sounds a lot like a vacuum leak too. The whole warm up enrichment was also what I was thinking. However, I have been pretty through and replaced old lines to check for leaks. I pressurized the intake and sprayed the all suspect areas with soap. I found a leak in the turbocharger compressor housing (but it leaked there and idled fine for years, I plugged it anyway. I found a small leak on a tapered plug I used to plug the evap system on the intake manifold (which i plugged in the first place because those lines looked crummy). There was no difference. Each vacuum line I have pulled off and plugged to see if one of them leaks sometimes just with a good finger (for break booster and climate vacuum). I replaced all the lines that went to the blow-by valve, the fpr, the turbo gauge, and the small line that went to the crankcase ventilation system. When I replaced that last line I had to clear a blockage in the nipple on the main crankcase ventilation pipe. I was thinking that it changed the fuel mixture and makes it seem like it has a leak, since I can't find anymore after some really deep searching. If I had a vacuum leak I should be able to compensate for it by changing the fuel mixture (using the LED). I still would not stabilize. The reactions to the changes seemed slow, so I'm thinking maybe a bad O2 sensor (it's the original). It tested fine, but I heard that the tests are in conclusive. I'm still open for more vacuum leak testing if you have any ideas. I tried a smoke test with a fog machine too.

So I'm going to try a pressure test and still look for vacuum leaks. I'm thinking about a new O2 sensor and performance wires and new plugs too. Thanks for your suggestions.

 

I did not realize your car was turbocharged. Is there any point (rpm) below which the turbos on these cars do not engage or are they always on? If always on, there is no way to have a vacuum leak and the conditions you describe are most likely caused by an inadequate fuel delivery condition.

 

Fuel pressure is not the issue if teh car runs fine any other time. Think about it. Fuel requirement is lowest at idle, and issue with fuel flow that affects idle will surely manifest at higher rpm's too, as more fuel is required.
My advice, if you've been living with it for five full years, why worry about it now?

 

That all makes good sense Typhoon. I'm still going to replace the wires, plugs and the O2 sensor, for good measure as well. I just really like it to run right. It does seem to idle with more stability since I plugged the last leak I did. Now it almost idles smooth until headlights and the heater motor is blowing.

Cycleboy, the turbo at an idle just free wheels basically. There is a charge air bypass valve (the noisy ones are called a hooter valve sometimes) that is open so the turbo can feed into itself. It makes it into a do-nothing machine at that point, but it's for when the throttle plate snaps shut. It won't continue charging air into the throttle plate and bounce back throwing the turbo into reverse. Back to the point, there is a vacuum condition when idling. I see what you were thinking, though.

 

Well, it's time for an update.

After 223k miles I replaced the O2 sensor. I haven't noticed any changes. I will be calculating my mileage since it's been down a little. I also decided to go with the IPD plug wires. They are a good spiral wound conductor. I like the tight fit. My old wires fell apart when I took them off (they were cheap carbon, low end brand wires). I even put in new plugs. #4 was the worst, but they all had almost double the recommended gap. They had at least 100k on those old copper plugs. They could even be the originals.

The verdict: it still hunts. Nothing seems to change its mind. I've checked from the inter-cooler to the head really thoroughly for leaks, so now I'll check more from there back. It's not leaking from any tube connections, since I have checked them all with carb cleaner.

So I'll keep looking for a leak, or I'll tear out the engine harness and split open the conduits to look at the wires. Then I rebuild them. Other than that I've checked it.

 

Titan Joe - what I've learned working on vehicles the past 25 years is that if a problem occurred after you worked on something then the problem is almost invariably directly related to what you worked on. You seem to have spent a lot of time and money investigating things other than going back to where you were before you started experiencing the problem. I don't mean to be rude or come off like a jerk, but that's what I would do.

 

I'm curious as to what fixes your problem as I have a high erractic idle/roughness problem that I can't figure out ether. Tried tune up, check for vacuum leaks with a smoke machine, did pumps because of the mileage, plus other stuff.

 

When I get this fixed, I will post about it.

Cycleboy- I agree totally. I took the intake manifold back off and checked the whole procedure and put it back on a few weeks after the initial repair. A few months ago I took it off again and put a better quality intake gasket and a throttle body gasket. I've checked the whole area for leaks with propane, carb cleaner, a makeshift smoke machine and soapy water with a pressurized system. I've even found a few really small ones. I fixed them all. I agree that it should be something that somehow relates the the initial repair, but I don't know what else to check. I've put in new injector seals and replaced all the vacuum lines too, but I'm could be missing something. Anyway, all suggestions are welcome, so please post whatever you think.

Also I have another thread where this problem has come up. It may be related to another very erratic misfire problem that recently cropped up. I thought it was relevant to mention this unsolved problem, and the subject of that thread has had some input on this hunting idle too. So here's a link to that thread too: https://volvoforums.com/forum/volvo-240-740-940-12/recent-development-heavy-misfire-50499/. I wish I could put all of the relevant info into one thread without getting the two confused, but for now people can check them both out.

Thanks for you posts. I'll keep you updated.

 

For me, the biggest clue to this mystery is that it goes away while the cold start circuit is closed. Once the excess fuel cycle shuts off, it starts to vary. That can only mean too much air/not enough fuel. What can cause this that's not a vacuum leak? Bad injector? Poor fuel pressure? I'm not sure.

 

I'm with whomever said, check the fuel pressure ... this requires a fuel pressure gauge which is connected inline between the fuel line and the injector rail. any shop that works on European cars should have the test gauge you need for this (the cheap testers only fit american cars, not Bosch managed Euro cars).

 

Could you confirm that the erratic idle is air /fuel related by feeding propane into MAF area?
When you had the intake manifold off did you confirm the injector spray pattern. My brother a VW mechanic he said he would get injectors that stream instead of spray that caused a stumble at idle that looked much like intake leaks.

That one dark plug could be from oil getting through the intake valve seal and guide, could it be leaking vacuum? You said you heard pressure escaping in the valve cover when pressure testing for intake vacuum leaks?

I am chasing a similar problem and I am going to try a shot of refrigerant then regulated compessed air and sniff with a refrigerant detector. This method has worked in the a/c world with nitrogen instead of air for moisture reasons.

 

I don't know why I did not read these earlier.

When the this problem first arose, I had a fuel pressure test done. It past all accept for one thing. The ball check valve was not working, so pressure would not be maintained after shut down. I just replaced it. No difference with the idle, but the car lights up faster from a hot engine start.

If I can get my hands on some gauges, I'll do the test anyway.

As far as leaking oil. I've had new plugs in for some time now. They look great.

I cleaned my injectors in a very ghetto way. I took an ultrasonic jewelry cleaning and put seafoam in it. I stuck the injectors heads into the cleaner and opened the injectors every now and then. I think I also blew them out with air while opening them. I have no clue how effective that was. It made no difference. I've run injector cleaner too. It made no difference. I don't have too much experience with them and I don't know if those injector cleaners do any good. Do you think I should have the spray tested?

I've still got a to check the intercooler really thouroughly (pressure and soapy water).

I may try propane into MAF to see if I can balance the mixture.

 

FWIW, my 92 740T occasionally stumbles at an idle. it will be idling fine, then miss and catch itself again and go back to a normal idle. fully warmed up and everything, just waiting at a light after smoothly gliding up to it. Might do this twice in one minute, might not do it for 2-3 days. i haven't figured it out yet, but maybe what you're seeing is an extreme version of this.

 

in school there was a really nice official injector flow bench/cleaner. really cool machine that's not used much at all in actual shops (well none that i've been in).

thinking back, I can't say that I can remember any car that ran crappy that we took the injectors out, clean them and the car ran better. It was more of a, people where putting there injectors in the machine because they where rebuilding there engine and they where out all ready and it's like why not. I had a few that cleaned up/sprayed better after cleaning, but I think they where injectors that I had sitting around my house. maybe a pintle was bent on one. maybe it helped a little.

I'd say I guess it couldn't hurt but I would say it probably won't help. if you think about it lets say all the injectors are lack luster spraying. it should go across the board. now if one was dead or really bad that should be able to be picked up.

 

Good to know. School's almost out for me. I'll finally have time to get into the intercooler and check for leaks. from there back. What I've done so far is got a spray bottle of soapy water and my air compressor plugged in where the blow-by valve is connected. I take off the small PCV line and connect the FPR line to it (disconnecting it from the FPR). Then I plug the line from the intercooler to the throttle body on the end that leads to the intercooler and pressurize. Then I spray the soapy water. I've found some really tiny leaks that way. But I need to check the other way too. I just don't want over pressure in the crankcase and blow out the crank seals.

We'll see what I can come up with.

 

I had a thread on turbobricks I started a few years before I joined here. A month ago I solved the hunting idle problem. I never snapped the new fuel injector caps on all the way. I thought they just fit loosely, but instead the gap collected some of the injector spray (leaning it) and would eventually drip (enriching it). The occasional inexplicable misfire at speed was evidence of the latter. In the meantime I've become an expert in the LH2.2 fuel injection system.