1988 Volvo 240 DL - Stalling and backfiring

Krampus · Post by **Krampus** » 04 Sep 2012, 13:15

Hello! A couple weeks back, I purchased a friend's 1988 240DL wagon which was on the way to the scrapyard after the MAF on it died and she decided it was finally time for a new car. It's become my project car, and I'm attempting to fix it up so that it can be my daily driver. I've got the idle smoothed out rather well, but I'm running into a few issues that I'm unsure as to where to go from here. I'm hoping my symptoms will sound familiar enough to someone so as to point me in the right direction.

The symptoms are that while the car seems to idle just fine, I get hesitation, poor acceleration, power dropouts, and backfire (I think in the throttle body) when I attempt to drive. Also, if I rev up the engine and then drop back to idle, it'll almost always stall. If I unplug the MAF, the stall doesn't occur. My first thought was that my idle is simply too low, so I tried to raise it using the air bypass thumbscrew on the throttle body. However, with the MAF is plugged in I can't get the idle up past about 500 RPM (I run out of screw first!).

The MAF is a junkyard reclaim that I've cleaned and tested, so I don't believe that to be the problem. The previous MAF burned out because the airbox thermostat had gotten stuck with the "hot" side open (at least that's my theory). I've cleaned the thermostat and it now opens/closes the box as it should (as near as I can tell), but I plan to remove the pre-heat hose altogether along with the thermostat and epoxy the "hot" side shut as common consensus on various boards is that it causes more harm than good. I also replaced the O2 sensor with another cleaned junkyard reclaim, but it shows a good swing between .1V and .9V so I don't believe that to be the problem. The throttle body itself has been cleaned both with CRC intake/throttle body cleaner (made a huge difference) and then yesterday with Seafoam (seemed to idle even better). Lucas fuel system cleaner has been added to the gas tank.

Looking through the Bentley manual, it's recommending re-adjusting the throttle sensor if the idle speed screw is unable to get the idle to where it needs to be. While I can do that, I'm concerned that I'm covering up another issue (since this car *used* to idle just fine, it seems weird that the throttle and sensor would get out of alignment). And even if the idle was too low, would that account for the backfiring? Can anyone come up with alternative theories as to what's going on?

Thanks!

- Jack

Post by **regent** » 04 Sep 2012, 13:34

1. I would verify the timing is right on the marks (just in case) and then check for vacuum leaks (a small leak would cause idle hesitation). With backfiring and poor accel, I would rather suspect timing.
2. Doesn't this engine have an IACM (Idle Air Control Motor) - they get stuck sometimes.

Do you know when the T-belt was last changed on this beauty?

Krampus · Post by **Krampus** » 04 Sep 2012, 13:59

I'll double check the timing next time I get an evening to work on it. I had that on the weekend list, but got sidetracked after discovering the airbox flapper was stuck.

However, I'm getting conflicting information about whether or not the timing can be manually adjusted. Some places say that rotating the distributor will advance/retard the timing, others say that it's computer controlled and can't be affected. The manual is annoyingly vague on this point. With regard to vacuum leaks, I actually replaced several hoses, although I know for sure that one was leaking. Running an unlit propane torch around the engine compartment near all the vacuum hoses I could see didn't seem to change the engine's speed. Do you know where I can find a diagram showing where all the vacuum lines connect? I hooked a vacuum gauge up to the line that ran to the cruise control servo and it was telling me... -15mmHg I think? I am skeptical of that, though, since I realized after opening the "new" MityVac kit that there was brake fluid all over it, so some of the seals may have been eaten away at.

I don't believe it has an IACM, just the manual thumbscrew on the throttle body.

The timing belt, plugs, and wires were changed "not that long ago." My friend that I bought it off of had a pretty decent mechanic who tended to the major issues, although unfortunately missed the airbox thermostat. I'd be surprised if they had screwed up the timing, which is why I hadn't been focusing on it, but now that I'm running out of major obvious things, I'll look into that.

Anyone have other ideas?

- Jack

Post by **regent** » 04 Sep 2012, 17:02

Sure thing, the electronics will compensate within a tooth or so of spark advance mismatch but if there is a disparity between crankshaft and camshaft, it is helpless.

Krampus · Post by **Krampus** » 05 Sep 2012, 23:24

I think you were totally right about the timing, Regent. The timing light showed it wasn't even close to the marks. I unbolted the distributor, however, and found that it was pretty well adhered to the base, so hand turning was out for now. I used a mallet and large screwdriver, along with liberal use of WD-40, to coax it into something at least within the timing mark area. I then managed to get it up to ~45mph without backfiring. Acceleration is crap and it hesitates, but it's much better than it was before. Good enough to prove the theory that it's as simple as bad timing. I soaked the bolt and flange with more WD40 so hopefully overnight it'll penetrate enough that I can turn it by hand. Otherwise I might try removing the whole distributor to clean out the gunk. Hopefully it won't come to that, as the plastic on it is super brittle and I damn near ripped the hall sensor off when i was losening the hold-down bolt.

I haven't had to adjust timing on a car for coming up on a decade, so allow me a dumb (probably easily googled) question if you wouldn't mind; If I want to set the timing to 12 deg advance and I've got a nice adjustable timing light, I can just dial in 12deg and align the mark with the zero mark, correct?

- Jack

Post by **regent** » 07 Sep 2012, 08:56

Sweet!
If I understand correctly, you've already verified the correct position of the marks on the crank and the cams, they are aligned and now you only want to tweak the spark advance by moving the sensor target at the distributor, right? If so, then you method with the light is fine. I just do not recall if the value was 12 deg though - it's been quite a while...

(Just make sure your crankshaft and cam(s) are 'in perfect agreement' before you go for it