I have a 1991 240 with 250k+ on it. When I have the hood popped open to do some work on it I notice that at idle the engine shakes violently, but if I give it gas it seems to go away after a second or two. Once the RPMs come down...back to the shake n bake. Also causes the exhaust to rattle like a cowbell. Engine mounts were replaced so that's not the problem. Any thoughts?

I was thinking 1) idle air control, 2) fuel injectors.

And other symptoms are that it is running rougher and rougher at low RPMs during driving.

 

3) bad spark plug(s) or spark plug wire or distributor cap

4) cylinder with bad compression

 

recently put in new plugs, wires and cap (copper plugs) so that's not it.

I think I'm going to try the idle air control valve...but before I replace can it be cleaned and if so how?

 

new IAC put in today and still no resolution to the problem. So now on to new fuel injectors...anyone have any recommendations?

 

You have a misfire. First take a compression test to eliminate serious reasons like a bad valve. If comp. is OK, then address ignition then fuel. Pull a spark plug wire see if there is spark on all four, etc. What plugs/wires did you use? Just because you changed them doesn't mean they are OK.

 

wow, people do not know how to diagnose these days.

2014 diagnosis = google your problem which maybe get lost is translation and believe what people say on the internet with such a passion Im totally adamant X part is wrong with my car regardless what others say. Then start replacing said parts.

No one reads books. There's so many good books on diagnostics and car repair and maintenance. No to mention repair manuals.

 

Well, if engine is in good tune (and you say it is)...

I usually check for air/vacuum leaks. (intake manifold gasket, vac lines, injector o-rings, hose that runs from air mass to throttle body). You can do this without replacing any expensive parts so it easy to rule this stuff out.

For intake manifold gasket i spray some carb cleaner or starter fluid around manifold and notice change in idle. (do this with caution and have extinguisher handy).

Visually inspect rubber hoses for intake and vac.

Lastly, i would pull throttle body and clean. 3 bolts, 1 gasket. pretty easy job. (less than $10 in cleaner + gasket). Also check continuity on throttle position switch.



If all of that fails, i would say idle speed motor, but you replaced it... so... I would check wiring from idle speed motor to ECU.

Hope this helps...

 

I'm borrowing a compression test gauge from a friend on Wednesday and will check the compression.

Something else interesting is that it is hard to start cold, and when I finally get it started it runs really really rough at low idle. But if I manually open up the throttle and take a screw driver and wedge it in there it keep the RPMs high it seems to run just fine. I stuck a screw driver in there last night and it ran for 5 min no problems. Then when I take the screw driver out it is back to sputtering.

When I tried to move it in the driveway it stalled. Restarted it.

Also, I am able to rotate the injectors - one with little effort, the others with a little more effort required.

Fortunately this is not my primary mode of transportation.

 

Well, yeah, all part of the misfire... A compression test will tell... Report back the readings... Let's see...

 

Ordered a Bentley manual today. $50. Figure it will be money well spent. I had a old Haynes manual but lost it during a move a few years ago.

Also, pulled the codes and got a coolant temp sensor code so I am in the process of trying to replace it. Got the part for $23 but need to spend some more $$$ on either a 3/4" stubby wrench or universal socket to allow me to remove the old one, it is not in the most accessible and convenient spot...anyone care to share their way of getting this thing out?

Compression test is still waiting for the gauge from a friend. He works as a dealership mechanic and forgot to bring it home with him at the end of the day.

 

You have access to an ohm meter? It might be easier to test the sensor in place rather than remove it. It's recommended that you take the intake manifold loose and pull it away from engine far enough to get access to the switch. Not hard, but I usually ohm test the sensor at the ecu plug. (you can get pin outs at volvowiringdiagrams.com)

 

To get to the CTS I use a longish extension or two with a 19mm? socket -- straight shot under the int. manifold. Very little coolant bleeds out.

 

Good news and bad news...

Good news first - Compression test was okay, and by okay I mean 137, 134, 130, 129. Used a Mac tools compression tool kit so feel confident in the results.

Bad news - decided to take throttle body apart and clean everything (also makes it easier to get to coolant temp sensor). Discover lots and lots of oil in system, and the oil separator appears to have a leak in it. Not too happy about this, the intake/throttle body/flame trap section is one I am usually on top of and take very good care of. The oil separator is weeping at the joint and has made my red brick into a black brick in that area. Not happy at all as this is going to be one PITA clean up job...embarrassed at the condition of the flame trap and intake system as like I said, I'm usually on top of it. The tube from the air filter had oil in it...I now have a really bad feeling about this car and am fearing the worse.

 

eh, brake kleen will take that oily mess right off the side of the block

 

Why bad news? The compression is OK, the other things are easy...

 

Compression is bad. That's really low, at if not just above min. spec.

 

The compression is fine as far as eliminating it as a reason for "violently shaking"... While not perfect, allowances could be made for other factors explaining it.

 

I don't have my books in front of me, but I believe its a little above minimum (i think min is around 125) and they are relatively close (no major differences between cylinders). I wouldn't worry over it too much, minimum is still 'good'. Fix the flametrap system and roll.

 

the bare minimum is no where near good in my book. 90ish wont start a car.

but yes, considering there equal isn't going to cause cylinder contribution issues,

 

128 is minimum
28 max difference.

9.8 compression.


I've done a lot more work on honda's and there min. is more like 150 and they only have 9.1 ish compression. usually produce 180-200 psi even with 200+k.

a volvo with 9.8 should put out over 200. You've lost 70+psi .