Just to start from the top, I've got a 1991 Volvo 940 with the B230FT. I just got done replacing the fuel filter and feed pump, some other deferred maintenance and hunted down vacuum leaks where I could find them. It now appears to start on the first crank, instead of the 2nd, and idles much more smoothly.

However, what I'm finding is that the idle speed in park is about 1100-1200, regardless of whether it's warmed up or not. When Shifting to reverse or drive, the high RPMs cause a decent clunk and then it vibrates a good deal. So I'm going to start by setting baseline RPMs with the options I have available to me. Based on preliminary research, RPMs should be 750 +/- 20, which sounds about right. I assume that's the RPMs in drive or reverse after being warmed up. If I'm wrong, feel free to correct me. The other figure I'm looking for is what the idle speed should be for a cold or warm engine in park.

If anyone has any additional information regarding idle speed figures I'm looking for or tips on setting baseline RPM, that'd be great. One clue I noticed is that while cleaning the throttle plate, I noticed it was open a bit with no pressure on the throttle spindle or cable, so that would suggest the idle screw was used as a crutch to keep the car running with the fuel pump on its last legs, right?

Anyway, thanks again!

 

a cold engine is around 1100-1200 before it drops to 700-ish when it warms up sufficiently. it should be 700-ish in park or gear, a/c on or off, if the idle air controller is working well.

 

Thanks for the info, pierce. Armed with that, I tried to get an idea of what's going on tonight.

I went through and manually confirmed no obvious vacuum leaks. I calibrated the throttle actuator lever to ensure that it was not keeping the throttle plate slightly ajar and ensured the stop screw was in the correct position. I also confirmed the TPS was calibrated in the correct position (I did not test it with a multimeter). For good measure, I cleaned the quick disconnects for the AMM, IAC and TPS. The following is what I discovered.

Regardless of whether it's cold or warmed up, the engine wants to idle at around 1100-1200 RPM and will idle at about 750-800 RPM in Drive or Reverse. I discovered that unplugging the IAC did nothing to the idle, positive or negative. After plugging the IAC back in, I unplugged the AMM and found that the idle lowered ever so slightly, maybe 100 RPM, as best as I could tell with my ears. If I plugged it in quickly, the idle resumed at the usual 1100-1200 RPM. However, I noticed that it hesitated briefly when I took too long to locate the pins when reconnecting it. So as a test, I allowed partial connection and it killed the engine. For what it's worth, with the IAC or AMM unplugged, although never simultaneously, there was no harm to the engine running when I applied throttle under the hood.

Obviously, there was some issue with fuel delivery before, because it will turn over the first time, cold or warm, and the vibration issues in Drive or Reverse are somewhat lessened. It would appear that there is something wrong with the base idle, but to think the IAC and AMM are both faulty (or at least don't help or hinder the engine) is remarkable to me. Frankly, I don't want to consider the possibility of replacing both, since I've been in the process of reconditioning the car for a test drive, which could mean more issues. Any suggestions are welcome as always.

 

next step would be to use a hose clamp tool to pinch off the hose going to the IAC, with that completely closed, the idle should drop to around 600rpm or less if the throttle is properly adjusted.


hot plugging the AMM/MAF is a bad idea, it can fry things.

 

Geez, I had no idea, I figured that was okay to do. If unplugging the IAC makes no difference, wouldn't disrupting the air flow to it make no difference? I'll give it a try regardless. Hopefully I didn't nuke my AMM or some other sensitive component.

 

Pull the IAC hose off and plug it. Check the TPS and make sure it is set correctly. Start the engine and adjust the throttle body screw to idle at 500rpm. This should be done with the engine at normal operating temp. After set then plug the IAC hose back in. Engine should now idle at around 650 rpm. If still high then either you have a large vacuum leak which could be in the IAC or somewhere else or the cold water temp sensor is bad or the throttle body is leaking past the throttle seat.

 

I didn't think there was any adjustment on LH 2.4 ?

 

Thanks for all the thoughtful input, everyone. Just to clarify, since I bought the 940, I have only been doing preventative and baseline maintenance. I haven't had the chance to drive it any more than on and off the trailer when I brought it home. So it was definitely a pig in the poke.

Anyway, I took the time to make sure the MAF was absolutely clean (even unbolted it from the air filter cover) and cleaned both ends and all wires with MAF cleaner. I re-checked all hoses providing vacuum to the intake manifold and the intake manifold gasket passed a cursory visual inspection, for whatever that's worth. Then I double checked and re-calibrated the TPS and idle stop screw to Volvo's specifications and I think it helped. Furthermore, I noticed the idle was wandering a tad, so I took a rubber mallet and gave the IAC valve a few taps and, wouldn't you know, it smoothed out. It took a few more applications of the hammer to keep the result consistent, but I think that was a factor in my issues. This was sort of puzzling to me, because one of the first things I did was take it off and give it a good bit of carb cleaner and used electrical cleaner on the contacts for good measure. I also got into the holes with a few q-tips to wipe off excess carbon/sludge.

Overall, the idle is a good deal smoother than before, but the only thing that perplexes me is that regardless if it's warm or hot, the engine wants to maintain 1100 RPMs +/- 100 RPMs in Park only. In Drive, it's just about spot on at 750 RPMs. The only thing I've noticed is that when it's in Park, before shifting into Drive to test engine load, the application of the brake pedal seems to cause a 100-150 RPM drop in idle speed. The hoses coming to and from the master cylinder appear to be secure and in decent shape, so I don't know if that's a factor. Lastly, the idle is also fairly smooth when in Drive, but I do hear a bit of metallic vibration, which I'm unsure if it's something like a heat shield or the exhaust, which does have a muffler leak. Lastly, I don't know how old the gas is, or if it's premium, but I put in the corresponding amount of Techron FI system cleaner, which probably helped somewhat. Fresh gas with the remaining cleaner may be of more help.

That's about all the new information I have right now. I plan on registering and insuring it tomorrow, so I may find out more then.

 

From the sounds of it your IAC is sticky. When in park, the IAC should reduce airflow to keep idle at 750 and increase it in park to maintain the same idle speed. I'd take it out again and inspect the little gate in it to see if it opens/closes without binding.

 

indeed, your description sounds like your idle is not being regulated, and defaults to way too high, which suggests its 'too open'. this can be due to the throttle itself letting too much air in, or the idle air valve being stuck open.

 

Thanks for the feedback, guys. It's easy enough to take off, so I'll inspect it again, along with its connecting hoses. I know the Bosch units are intended to be sealed units, but is there any way I can more thoroughly clean/inspect the IAC than I did before? Worst case, I'll just have to find a replacement. Speaking of which, I don't suppose the IAC on my 960 would substitute as a troubleshooting comparison, would it?

 

Your statement of your idle dropping when you apply the brakes may be an indication of a bad check valve or a tear in the brake booster.

 

Thanks for pointing me back to the IAC, everyone. I gave it a very thorough cleaning and the valve moves MUCH more easily. I will reinstall it tomorrow after checking the hoses for tears. If there's no result, I'll just have to source a replacement.

Thank you for the suggestion. I was wondering what the cause was. It looks like I can source a replacement for about $25-38.