dr bob

Bill--

Do you have the intake pressure/vacuum test setup used to verify intake integrity after refresh? Eliminates hoses and other vacuum leak issues quickly.

Jim Morton

Agree with Dr Bob about putting on Vac guage on it. Could tell the story...

dr bob

The thing that steers me away from the ISV is the original symptom that Bill describes: "While turning to get on a freeway on-ramp last night, my trusty 89 stumbled, almost died, stumbled a bit more, feeling like it had lost power, then finally responded to the throttle and seemed almost normal accelerating and cruising at freeway speeds." Says mixture problem to me at least initially. As soon as the throttle is opened, the ISV is out of the picture. Fuel starvation is most apparent under higher load/higher RPM's when fuel needs are highest. Meanwhile, air dilution is most apparent at idle, when mabifold pressure is lowest and the mass of leaking air is highest in proportion to the air ingested by the engine. Hence the suggestion to look first at possible air leaks. Still not a bad idea to look early at fuel pressure, since the pump was recently replaced.

Ignition doesn't get ruled out, since it's apparent at low RPM's/high load (launching onto the freeway) but is masked by higher RPM's and low loads while freeway cruising.



Curious what the plugs look like. They are usually pretty good initial indicators of combustion problems.

Bill Ball

Thanks Dr. Bob and Jim. I need to retrieve my vac gauge and my intake tester, but finger over a small vac hose end shows it's sucking some as long as the revs are not super low.

ISV test - definite "clack" - it's moving.

No gas or gas smell in damper or FPR vac lines.

Unplugging and replugging MAF - no change.

BIG CLUE: Help me reason this out. If I place my hand close to the MAF entrance somewhat reducing air intake, the engine revs smoothly and higher as I get closer until I almost shut off air flow, then it dies. With hand just right it idles seemingly normal.

I've tried to sort through what that means, but my basic logic is failing me. With a carb, that would indicate a lean mixture and I am richening it by restricting airflow. With a MAF, I get confused as to what this indicates. I don't have to get real close to get the revs to zoom and smooth out. I hover an inch or two over it.

Again, it revs very well and as quickly as ever once I get out of the bad idle zone.

Additional clue: I tried to run the idle stabilization routine, and it struggled but failed to achieve anything, and the engine died after 15 seconds or so. And while I was running it, I smelled gas and the exhaust looked slightly black. So, I thought, for some reason it is rich, even though it starts perfectly before the idle drops and becomes rough. That prompted me to check the dampers. If there is a damper leak, I can't detect it. I will detach and plug the vac line and see if anything changes. I detached but did not plug them, creating a bit of a vac leak with no change in the idle.

Chalkboss

Perhaps don't get too caught up in trying to figure out the MAF hand-waving thing (at least for me). I had rough running and did this very same thing. In my case the culprit was one injector harness not seated fully- one cylinder getting no fuel. I puzzled over the MAF too...

Rob Edwards

Would you like a spare MAF to be placed in the mail to you for testing purposes? I can't reason my way through diagnostics, but I can throw spare parts at them....

Leon Speed

Here's a stab in the dark: remove the injector leads one at a time to see if ilde changes.

jcorenman

Bill,

I think you need to try another MAF. When you wave your hand near the MAF inlet, you are changing the airflow distribution, with more or less air going through the central venturi where the hotwire is. That will make the mixture richer or leaner, depending on how the airflow is shifted.

MAF's usually read "lean" as they age, but they can also fail and read "rich". We had an intermittent problem with our GT that sounded very similar to yours. It turned out to be running stupid-rich at idle, beyond what the LH could adapt, and was the MAF. Louie got us a JDS rebuilt and viola!

Call Louie, he usually has MAF's in stock and might be able to send one on trial. He can also test your MAF. Louie's phone# is here.

Cheers, Jim

jorj7

Bill,

I think I have a spare MAF in one of my bins. I can check it out tonight.

__________________
George
90 S4 Grand Prix White (Murf #5)
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http://928.jorj7.com

dr bob

Bill--

I'm still thinking vacuum leak someplace, since a failed MAF wouldn't allow much power past the 'limp off the freeway' default mixture the LH uses on no MAF signal. Of course, that also fits your symptoms for MAF failed. Simple enough to swap one in if Jorg knows where his is stored.

Injectors/connectors/wiring test from the protection relay socket with a DMM. 4 Ohms each one, four in parallel in each loop. Let's see, multiply by the square root of three, add in the time of day and air temperature (in ºK), and you should see the same resistance on each loop. An open or loose injector sticks out like a sore thumb with the higher loop impedance. Shorted wiring shows up with low or no loop resistance.

Mrmerlin

FWIW you probably know all of the things to do that i have posted but its for the benefit of others that might find this thread in the future.

That said,

Bill with the new info I would still recommend that you swap out the MAF,
make sure to do a battery reset as well , the new MAF will change the running parameters.


Also inspect the flappy valve to make sure the side with vacuum will hold and the side that goes to the flappy has a vent to atmosphere .

Do a flappy test , put a piece of tape on the flappy shaft and start the car,
the flappy shaft should run to full close then then back to open just as the engine is started.

Inspect the 2 plastic connectors that plug into either side of the MAF boot.
These plastic connectors can crack and crumble thus causing an air leak,
they seem pretty weak from oil damage

jcorenman

Not necessarily. MAF failures come in a variety of flavors. The most common is a progressively lower output (and lean fueling) with age, because of erosion of the hot-wire element. And as you mention they can also fail outright with no output-- which triggers the "limp" mode. But it is an analog circuit, and can also fail in ways that cause a high or low output. Not common, but mystifying when it happens.

Bill Ball

OK, so a MAF can fail and only affect idle? It revs very freely and is fine on the road except at stop signs. There is a problem in neutral trying to hold 2K RPM, it initially revs to 2K with only a small amount of throttle , but then is drops unless I apply more gas and feather it. But above 2K or under a load, it seems normal. So is it possible the MAF is lean at idle but OK at higher airflows? The anomaly of the idle improving with my hand hovering over the MAF opening puzzled me.

I can't see an injector problem only being idle or just off idle only. It would have to have a lot of bad injectors for the idle to be so poor, and then I would expect it to run poorly at higher RPM too.

I still need to check for a vacuum leak. I checked all the hoses I could see, and the MAF boot is seated. Nothing apparent. And I am puzzled that it only affects idle. With a vacuum leak I would expect surging, high idle, but I suppose a massive leak would not. Again, odd that only idle.

Sorry if I sound argumentative, but nothing makes any real sense to me.

Bill Ball

Heh. I do that a lot too. I think George has me covered or I can get the one I loaned out back. Thanks Rob.

Bill Ball

Stan - Flappy works. I'll take a closer look at the MAF boot plastic connectors.