Why does my engine seem to choke when I bump the throttle?

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.

Any ideas?

I just had the timing belt and all seals redone with new crankshaft
bearings, too.

Does this sound like timing? Vacuum? Fuel Filter? Other?

Thanks!
Jamie

 

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.

Bob

 

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.

 

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.

Mike

 

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.

Thanks!

 

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.

Mike

 

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.