1987 740 GLE B230F
OK, engine starts and idles just fine - sounds good. While idling I either turn the throttle assembly by hand under the hood 1/4 turn or barely push the throttle. By doing this the engine seems to choke out for about 1-2 seconds then picks up with a normal fast idle.
I did this repeatedly, barely turn the throttle, the rpms dip, then pick back up.
Idles -- dips -- picks up.
Instead of going from idle to fast idle, it's like it chokes a moment then goes.
I just had the timing belt and all seals redone with new crankshaft bearings, too.
Does this sound like timing? Vacuum? Fuel Filter? Other?
Could be the IAC isn't opening all the way or is sluggish. Could be the MAF is going south. Clean the throttle housing and spray out the idle motor, and see if it improves.
Two other things I noticed:
I am guess the MAF is the electronic assembly just past the air filter. (Square metal box in the intake line?). If so, is this supposed to be hot? After running around the neighborhood, this was hot to touch.
Also, traveling from the air filter toward the intake manifold, just beneath the throttle assembly are 2 vacuum lines in the intake. A small hose on the right and a larger on the left. The left hose kept popping off and was split open. So, I cut the end, reattached it and after another run it popped off again. So I cut it again and it did it AGAIN.
So, I ended up using a tie-wrap to hold it on and it hasn't come off since.
No - it shouldn't be hot. Feel the top of the air cleaner box... it shouldn't be hot either. If it is, open the box and look at the flap that controls whether the air comes from outside or from the preheat hose. You will probably find it bringing in air from the preheat hose. It is controlled by a small pellet down at the bottom; they fail frequently and pretty much always fail in the "hot" position. You can remove the preheat hose as a patch until you replace the pellet if it is bad.
Those hoses sound like the pair that go to the charcoal canister. Mine don't like to stay on either but that doesn't seem to affect engine operation.
Hmm, I wonder if that's why it feels like hot air is being pumped in the car when I am driving. Maybe something different all together.
I'll need to look at my Haynes manual about that pellet - I am not sure I follow your description, the air box I don't think is hot, but I'll check.
Yes, I do recall the 2 hoses going to a cannister near the headlight on one end, and the intake on the other - near the intake manifold.
Update: I removed the airbox and saw the flap that directs air flow. There is a hole below the flap that I inserted a screw in and this held the flap open and directed fresh air into the intake - thus closing off the hot air intake.
This made the car run like crap - not idle bad, but it ran very underpowered.
So, I removed this and it seemed to default back to the hot air intake, but the car ran better. So, I don't know if this flap is changing on its own, or only letting hot exhaust air into the intake all of the time.
That's odd. It should run fine with the hot air intake closed off. Maybe the cold air intake is blocked? I've never looked closely at how it is routed.
Thanks! Please see my timing belt post above. Also, there was a hole about 1/4" in the intake hose between the AMM and throttle body.
Sorry Jamie, meant to send to group on first send
I myself really don't know but in looking at the FAQ's I saw this which might have something to do with your problem or at the very least looks like a quick easy test to eliminate one cause.
[Diagnosis 5:] The fuel pressure regulator is worth a careful inspection. The fuel pressure must rise instantly in response to the vacuum signal fall that accompanies a throttle opening. A hardened diaphragm might be causing the fuel pressure that has been lessened by the fuel pressure regulator to not increase as rapidly as it must and you won't get the appropriate fuel quantity in spite of lengthened injector duration. Try an acceleration test with the vacuum hose pulled off fuel pressure regulator. Easiest way to check the FPR is to pull the vacuum hose off of it while the engine is idling. If the idle picks up, your FPR is good.