When I finished up a previous thread on this issue, I thought I had solved my problem but only had a minor issue. Turns out that minor issue still causes stalling.

History:
1993 240 wagon. The car is relatively new to me. It ran fine at regular speeds but would stall when in stop-and-go traffic or when just barely moving along, such as in a parking lot or a crowded neighborhood street with lots of speed bumps and you have to carefully squeeze past cars a lot. I live in a very overcrowded part of Oakland, CA and commute through all surface streets to Alameda, CA.

I thought it was a low fuel pressure issue but now can't be so sure. The in-tank fuel pump was not running at all so I replaced it. I also replaced the fuel pressure regulator. That did not change what my fuel-pressure-gauge said, so I no longer trust the gauge. The main fuel pump can easily get the pressure up to 90 psi, so I am pretty sure it is fine, and the fuel filter looks brand new.

I have cleaned the throttle-body and idle-air-valve and verified the operation of the idle-air-valve. I had discovered that the idle switch was maladjusted and was never closing. So, I adjusted the throttle-valve-stop-screw, the position of the idle-switch, and the throttle-linkage according to the recommendations in my Bentley manual.

The Issue:
After adjusting everything: I noticed that, when I turned the throttle-pulley by hand, just as the idle switch would click open (thus closing the idle-air-valve), there would be a drop in RPMs until the throttle was opened a bit further to let in enough air to keep the RPMs up. It seemed as if the idle-switch was simply opening too soon. However, everyone says the switch should click open the instant you open the throttle-valve even a hair. In addition, my particular idle-switch does not allow the throttle-valve to travel any more than a hair (in the closed direction) past where the switch itself clicks closed.

So, I thought I would try adjusting the throttle-valve-stop-screw a bit to open up the throttle-valve a bit more (also adjusting the idle-switch and the throttle-linkage to match), thus allowing more air to pass at the point when the idle-switch opens. Unfortunately, there are limits to how far one can set the throttle-valve-stop-screw before it causes the engine to over-rev when you are trying to sit still at idle.

My problem is that there seems to be no point where I eliminate the RPM drop (just when the idle-switch opens as the throttle-valve is opening) but without causing way too much over-revving. In the situations where I have to drive, I spend a lot of time with my foot only barely on the gas, just creeping along. (Yeah, I hate it, but what are you gonna do?) So, I spend a lot of time with the throttle-valve going just between completely against the throttle-valve-stop-screw and just barely open and back again. Constantly hovering in and out of that reduced RPM zone. Eventually, the thing will stall. But, as before, it only does so after the engine has warmed up.

Here's my Question:
Is it possible that my idle-switch is old and worn out? That the switch has been mashed closed so many times that it doesn't allow for the throttle-valve to open as far as it should before the switch opens? In other words, if I got a new idle-switch do you think it would have more "play" between when it clicks closed and when it just won't let the shaft turn any further? Mine only has like half a degree of "play." Is that completely normal?

If you think my idle-switch is just fine, then what else could be the problem? Why are the RPMs dropping for that short instant just as the idle-switch is opening?


Thanks

P.S. I know this is a long and detailed question, but my problem is a little more specific than "Umm it won't run." I appreciate any help anyone is willing to give.

 

I've been battling the same issue on my 1992 240 wagon. Here's what I've done so far; I'll list and give details on the effect it had.
-Cleaned Throttle Body - you said you did this already, as have I, helped slightly but not much.
-Air Mass Meter - I've cleaned mine, which helped a bit. It's still sluggish, but comes out of it quicker. Also, I've noticed that my car runs better without the air mass meter and o2 sensor plugged in. Perhaps you could unplug them, and see how it runs. I've ordered a replacement to see if it'll help.
Have you checked the on board diagnostic box for codes?
Basically, see how it runs without those sensors.
Hope something works out!

 

Oh, I forgot to mention, that I also already cleaned my mass-air-flow-sensor (AKA air-mass-meter). That helped a bit on mine as well. This was even before I fixed the adjustment on the idle-switch.

As far as replacing the mass-air-flow-sensor goes: I have read elsewhere in this forum that unplugging either the MAF or the O2 sensor causes the ECU to go into "limp home" mode with default settings. If the car runs better, it may not necessarily be because of what you unplugged, but merely because of the limp-home mode.

So far I have found and fixed two things that were completely wrong on this thing and replaced one part that may or may not have needed to be replaced. I hate to go in and start replacing really expensive parts for no reason other than shot-gun troubleshooting.

I am about ready to give up and take it into the shop.

 

I've gotten to the point that that's what I'm doing. After the air mass meter I'm probably going to replace the o2 sensor. After that perhaps fuel pressure regulator

 

I doubt that your idle switch is worn out. If I remember correctly (and I am going by memory since my 240 is away this week) that it should click at the slightest opening of the throttle body. The procedure you went through to adjust it ensures this.

Have you checked your air intake system upstream of the throttle body to the AMM for any leaks? Any air that leaks in doesn't get metered by the AMM and therefore can cause issues when opening the throttle.

Also, it very well could be your AMM going bad. I try to keep a spare handy to rule this out when debugging like this. I like to pick them up in the junkyard whenever I see one that I suspect will be good (like on a wrecked car). So far I have had good luck and usually spend around $30 for one. I test them immediately (like, in the parking lot of the JY). The one by me allows exchange for equivalent parts if the purchased part doesn't work.

Frankly, with the work you put in on chasing this issue, I doubt you will have much luck taking it into a shop unless you know of one that specializes in Volvos and has someone around that still knows how to work on these older models. Keep pushing, you'll solve the problem.

 

indeed, air leaks between the the AMM and the engine are a prime suspect.

 

So, I took it to a shop today. It is a Volvo shop that has been on Alameda (where old Volvos go in their afterlife) for ages. He told me that the idle switch was broken. That there should, if fact, be play between when the switch closes and when it can travel no further. (This was before I mentioned that I had been suspicious of this exact thing.)

The fact that one should adjust the switch so that it opens just as the throttle opens is an entirely different parameter from how much play there should be once it has closed.

In addition, the shop told me that the rear two spark plug wires were barely connected, that the plugs are shot, that the distributor rotor and cap are about shot, that the plug wires are really old and should be replaced, and that the brakes the previous owner (the other Volvo shop in Alameda) told me were new are actually almost gone.

So, this shop replaced and adjusted the idle-switch. Oddly, though, they did not adjust the setting of the throttle-valve-stop-screw or the throttle-linkage. So I will do all that this weekend. Yes, it all cost me $311 (mostly for troubleshooting), but I also got valuable information which saved me some time. I probably paid too much, but I am getting used to that here in the Bay Area.

All the rest of the stuff, I can replace myself. The shop owner even offered to sell me the parts so I could just replace them myself. But I will probably order them on-line. I have given him enough money for now.

 

I have checked for those, but not since I cleaned the AMM so maybe it wouldn't hurt to check again.

 

nah, fix the ignition first, if its thtat bad. I would have bought the wires and plugs and stuff from him on the spot, I bet he's not THAT much more than internet, and you don't have to pay shipping and wait a week or two. support your local gurus, etc etc.

 

I may still do that. He is almost right on my way home from work.

 

Well, it turns out that all this guy did is sell me a part that I didn't need and adjust the throttle-valve-stop-screw even further in so the car thinks I have my foot on the gas all the damned time.

1. He told me that he did not adjust "any of that stuff" but I know exactly where I left the throttle-valve-stop-screw and it was several turns further in when I checked it this morning.
2. The way he described the problem with the idle-switch made it sould as if I was correct about there needing to be more play between when the switch made contact and when it would no longer allow the throttle-valve-shaft to turn. This new idle-switch behaves exactly like the old one.
3. He didn't even tighten the rear mounting screw for the idle-switch all the way. When I went to adjust it this morning, it was loose.
4. When I put all the adjustments back where they belong, it still has the exact same problem.

So this guy is obviously one of those mechanics who just does whatever will make the initial symptom go away without figuring out what is really wrong.

I am back to just doing a standard tuneup and guessing my way through from there. I have ordered some of the parts I need and will order the rest soon.