1996 Mzada 626 basic RPM at startup too high

LEN

MEMBER
1996 MAZDA 626
4 CYL
FWD
MANUAL
473,000 MILES

Hey All,

Had a very rough running Mazda 626 on LPG, engine code FP. After renewing the spark-plug cables, some vacuum hoses and cleaning the throttle valve, I installed the lot again and got it running. It ran at a very fast basic rpm speed. (Rpm was 2500) I tried to screw this down with the air screw under the throttle valve. That did not work.
All of a sudden after some trial runs of about 5o km (about a week in total) the basic rpm dropped to about the normal 650-700 rpm. Only at cold start up it still runs at 2500 rpm. New "feature" on this issue is that after warming up the rpm drops too slow to the basic rpm (700) which makes shifting gears a problem.
BTW all symptoms while running on petrol and on LPG. The engine seemed to like what I did. It runs very smooth and the power has come back to it. It just doesn't like the way I adjusted the basic rpm. I don't know however, what I did wrong.

Has anyone any clue as to what I did wrong when reassembling all these parts.
The parts that came loose were. Throttle valve, petrol pressure thingy that is located on top which regulates petrol pressure when accelerating, and the vacuum valves that are connected to it. I fastened the map sensor which had come loose for some unknown reason. All vacuum hoses are checked. The spark plug cables are renewed. I further disengaged the battery every time I got to work on the engine.

Thanks in advance for your answers.

5 Replies

KHLOW2008

MECHANIC
41,814 POSTS

After cleaning the throttle body etc, the engine rpm would definitely increase as air flow is now increased and you need to lower the engine rpm.

When adjusting the basic idling speed, it should be done after the engine is at operating temperature, anything before that would result in the engine rpm dropping too low after it is fully warmed up.

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Monday, April 18th, 2011 AT 5:05 PM

LEN

MEMBER
3 POSTS

Thanks for the answer KHLow2008,

You are right, I didn't regulate the idle rpm at operating temperature.
Does this also explain the slow descent of the rpm's to idle when the throttle closes (or should be closed). Further, is there an explanation for the quit sudden drop to a normal idle speed after some trial runs with no real interference from my part.

Again thanks in advance for your answers.

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Tuesday, April 19th, 2011 AT 7:23 AM

KHLOW2008

MECHANIC
41,814 POSTS

The idle control is temperature related and as the engine heats up, the Engine Temperature Sensor would monitor and send the data to the Engine Control Unit (ECU) which would lower the idling speed according to conditions. If coolant in system is insufficient or the ECT is dirty/faulty, it would affect this.

During throttle closing, especially during sudden closing of the throttle, the idling speed would drop to about 1000 rpm before gradually dropping to the normal idling speed. It is designed thus to prevent the engine speed to drop normal too fast as it can over shoot to lower than the idling which would affect drieveability and might even stall.

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Tuesday, April 19th, 2011 AT 6:49 PM

LEN

MEMBER
3 POSTS

Hey KHLow2008,
Thanks for elaborating on this subject. It is clear to me now what happened after I reassembled the parts after cleaning. The related articles helped me a bunch too.
Great site, nice experts.

Len

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Wednesday, April 20th, 2011 AT 8:50 AM

KHLOW2008