Engine misfires when decelerating from high speed

HELP. My mechanic is baffled and after $1,200 worth of work the problem still occurs. My mechanic has been test driving the car and cannot reproduce this problem.

1999 Subaru Outback Legacy. 140,000 miles, standard transmission.

This problem only occurs (so far) when I am driving at high speed, in 5th gear, like on a highway. And it does NOT happen in EVERY instance. I?m traveling at high speed and then need to exit or stop, so I slow down by just easing off the accelerator and then eventually depressing the clutch and applying the brake. (Don?t downshift unless it?s a REALLY long exit ramp.) So when the clutch is depressed and I am slowing–and thus not giving it any gas-- the RPMs come down very quickly. At this point the engine will start to misfire and suddenly behave as if it is running on 3 cylinders. Noisy, very poor acceleration from gear to gear. Very rough. The engine does not always completely stall. The only way this is ?fixed? is if I turn off the engine and restart it. It always restarts, and then runs normally. This doesn?t happen during normal city driving on city streets. Only at high speeds with (relatively) quick deceleration AND in neutral, like when using a highway exit.

So I had a tune-up (including new plug wires) plus had the ignition coil AND the idle control valve replaced. New clutch installed a year ago at about 130,000 mi. I have been driving stick shifts for almost 30 years so I don?t think it?s me.

When this occurs the CHECK ENGINE light goes on. When I have this checked at the garage, the computer shows an idle speed malfunction, but as noted I had the idle control valve replaced. When they check the idle speed in these instances the idle is functioning in normal parameters. What the computer is saying doesn?t seem to match all the ?symptoms,? so the real source of the problem seems to be something that the computer doesn?t read.

Any ideas?

Something is happening under very high vacuum conditions.

Perhaps either the fuel pressure regulator is pulling all the way and sticking, or perhaps something is opening a vacuum leak that somehow closes again when the car shuts off.

I can’t imagine where there could be a vacuum leak that would do this, but big vacuum leaks will set a code for idle control.

My thoughts are also along the lines of a vacuum leak that is very small, but is aggravated by the sudden decrease in engine speed.

I’m guessing, but if the IAC air circuit is gummed up, and the IAC cannot close completely, as it should when decellerating with the engine RPMs above idle, you could be getting a lean mis-fire. The cylinders are still getting air, but the ECM is cutting the fuel off. Removing the throttle body and seriously cleaning it out may fix this. The tip off to me is the trouble code pointing to problems with the IAC. The old one was probably not bad. The trouble code is circuit problems or parameters out of range. A bad connector and gummed up air circuits can also cause these problems.

Similar thing happens to my car. I have a Toyota Corolla 1997, 51,000 miles on it.

Engine stalled twice at highway toll, and twice at the exit ramp. For a couple of months it was perfectly fine in city low speed driving. However, I have experienced it twice in past week, accompanied by rough idling . One of them was very rainy day.

Yesterday, I took it to my mechanic which is 9 miles away. First half of the road, engine ran very rough. Only the way, I could keep it idling normal was giving more gas. However, before I reached my mechanic car start running fine. My mechanic test drove it, and car was fine all the time. He gave me a half a dozen reasons that my cause it, but refuse to fix anything because there was nothing obvious. Following is an example he gave: if a part such as fuel pomp is defective it may work fine most of the time, but have problem maybe 5% of the time. That when car have problem. If such a thing is true, then it will be difficult to troubleshoot it, since this can be true for almost any part.

We also checked car computer, but it was fine, we did not detect any trouble code. Whatever it is computer cannot detect it.

I left the car with my mechanic, he wants to use the car for several days and see if it happens again. Since problem is very subtle, it is hard to diagnose. I hope he can figure it out. But, I have serious doubts at this time. I think someone can only diagnose when problem become much worse.

At the moment only thing I could do was be an AAA member. I know I would get stuck one day with this car somewhere.

If my mechanic figure out what is wrong with my car, I will post it here.
Good luck!

Check the intake manifold for vacuum leaks.

Dear Eyigit:

Boy this sound familiar. I’m worried about using the highway, which is very often NOT a practical attitude. Thanks for the support!