Post-rebuild, stalls on throttle release in closed loop (vacuum gauge video)

[Jim Chandler]

Hi all you friendly, helpful folks!

This is on a 1996 Mazda 626 2.0L Manual trans.

I recently completed an engine rebuild, due to a worn cylinder head (leaking valves, one lobe worn down, etc.) and high mileage (246,000)

The build went smoothly, and the engine started right up, however it won’t hold idle properly when entering closed loop (after warming up to operating temp.)

I’ve checked TPS signal, IAC signal, timing is set correctly (12deg BTDC base timing), no exhaust leaks (replaced a leaking downpipe while engine was out) O2 sensors reading properly, I’ve dissabled the EGR valve, and I don’t detect any vacuum leaks. I’m not sure what else to try, and looking for any input from all you smart folks out there.

I recorded a video of the issue, while watching a vacuum gauge. Around 2:00 it changes from Open to Closed Loop (warmed up) and the difference becomes quite noticeable. I could not attach the vacuum gauge video directly, however I’ve created a YouTube link, hopefully it works: https://youtu.be/DbNlRavKCcQ

EDIT: I should have mentioned, in the video I let it idle from cold to warm, then (after 2:00) I run the throttle up three times in the video. First, a short run to 2500rpm, and release, to show a stumbling and recovering condition (ECU compensating for the problem, whatever it is) Then two more throttle-ups, where I run for a good 10-15 seconds, showing the stall.

EDIT #2: The background hissing noise between the first stall and second stall is just static from my over-eager radio that likes to come on automatically when I start the car.

Thanks!
-Jim

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[Jim Chandler]

After posting, this afternoon I did the following:

– SeaFoam thru the TB boot, + gas treatment in my half-tank. It belched out a LOT of smoke, but no change in throttle-stall condition.
– The rebuilt engine is still not broken in, so I drove 40 miles on the freeway, alternating WOT and closed throttle (to the annoyance of all the other drivers on the freeway, sorry fellas!) Total miles on the engine so far: probably 50. (It runs fantastic on the freeway, fwiw)
– Compression-check after driving: 122, 120, 115, 123

But alas, it still stalls.

Perhaps this could be related to the ECU re-learning things post-rebuild? I thought I had cleared it by having the battery detached during the rebuild process. In any case, I wouldn’t think it would take this long to re-learn if that were the case. But maybe I’m wrong…

[Jim Chandler]

Today, I cleared the ECU by disconnecting the battery, then holding the brake pedal for 30 sec., drove it for a while, but no change in the condition.

Then, finally, with no other ideas, I decided to “tamper with the idle set screw” in the throttle body. Now, I understand this is set at the factory, and should never be adjusted, according to many sources out there, but a thought occurred to me; It’s a working theory, really, but if the set screw is configured at the factory, for a specific volumetric displacement of the stock engine, then perhaps changing that displacement (as with my case, boring 0.020″ over) necessitates an adjustment in the set screw.

So, I put it in diagnostic mode (where it didn’t run initially) then backed the set screw out a few turns, started it up, then fine-tuned the set screw until I reached 750rpm. At this point, though still a bit unsteady, it holds a decent idle and no longer stalls.

I’m not sure if this was the correct fix or not, but it was the only option left – if anyone knows anything further about these idle set screws, perhaps you can advise if my reasoning is correct – that I should actually change it when the engine displacement is changed like this.

[Author]

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