Surge or dip at idle?
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Joined: Oct 2023
Posts: 3
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Surge or dip at idle?
1989 Chevy c1500 with 5.7 and 700r4 transmission.
been working on a truck i got. Friend and I replaced the head gasket because it was blown.
the main thing was a blown head gasket. my friend and i got that taken care of.
1.We took the engine apart and replaced all the gaskets and seals and stuff.
Some would call this a reseal + accessories.
2. we replaced the timing chain and water pump at the same time.
3. new distributor, wires, spark plugs and coil
4. new IAC
5. New fuel pump
6. new starter
7. new fly wheel
8. new rear main seal
9. replace a bunch of broken electrical connectors
10. new coolant hoses.
- we did the computer relearn on the timing and IAC and anything else that needed it.
The truck start and runs great. we drove it around the block and it shifts wonderfully ( previous owner installed new ( rebuilt ) transmission ).
My friend even thinks it is way "stronger" than his 5.7 sierra. that does not matter. But it drives fine and does not die and goes when you want.
basically, my friend said the truck is healthy.
*** the issue ***
The truck has a very slight surge at idle. maybe its a dip at idle. I do not have a tach but it sounds like its just a few hundred RPM dip every 10 seconds or so. This is just based off of my ear. so take that how you will.
But it start right up, idles fine other than the slight surge and it never dies and you can rev it with no bogging down and you can blip the throttle and it again revs and does not bog down and goes back to there it should.
*** the question ***
With all the parts that have been replaces and the work that has been done, where should i look next?
would a slight idle down be electrical, mechanical, fuel related, spark related, computer/sensor related or what?
we checked for vacuum leaks and did not find any.
just looking for a next step to check.
I do not have any blink codes or anything.
been working on a truck i got. Friend and I replaced the head gasket because it was blown.
the main thing was a blown head gasket. my friend and i got that taken care of.
1.We took the engine apart and replaced all the gaskets and seals and stuff.
Some would call this a reseal + accessories.
2. we replaced the timing chain and water pump at the same time.
3. new distributor, wires, spark plugs and coil
4. new IAC
5. New fuel pump
6. new starter
7. new fly wheel
8. new rear main seal
9. replace a bunch of broken electrical connectors
10. new coolant hoses.
- we did the computer relearn on the timing and IAC and anything else that needed it.
The truck start and runs great. we drove it around the block and it shifts wonderfully ( previous owner installed new ( rebuilt ) transmission ).
My friend even thinks it is way "stronger" than his 5.7 sierra. that does not matter. But it drives fine and does not die and goes when you want.
basically, my friend said the truck is healthy.
*** the issue ***
The truck has a very slight surge at idle. maybe its a dip at idle. I do not have a tach but it sounds like its just a few hundred RPM dip every 10 seconds or so. This is just based off of my ear. so take that how you will.
But it start right up, idles fine other than the slight surge and it never dies and you can rev it with no bogging down and you can blip the throttle and it again revs and does not bog down and goes back to there it should.
*** the question ***
With all the parts that have been replaces and the work that has been done, where should i look next?
would a slight idle down be electrical, mechanical, fuel related, spark related, computer/sensor related or what?
we checked for vacuum leaks and did not find any.
just looking for a next step to check.
I do not have any blink codes or anything.
CF Veteran
Joined: May 2011
Posts: 1,578
Likes: 302
From: Central Iowa
If you’re still needing an answer….
It’s probably just my fatigued ear drums, but I don’t hear any surging in that video.
To verify if the RPM’s are actually changing, hook up an auxiliary tach or use a timing light with a tach function.
Surging at idle with no driveability concerns could be a vacuum leak; fuel supply, idle control or timing issue; or a torque converter/clutch pack that’s not fully releasing.
If you confirm a change in RPM, there’s a few checks to make with the engine at operating temperature and idling:
1) Disconnect the spark control circuit and check engine speed - if it stabilizes, there’s an issue with spark control, whether it’s distributor position or related sensors (cam and crank).
2) Unhook and cap/plug all external vacuum ports (brake booster, PCV, purge valve, MAP sensor, etc). If idle is stable, hook them up one at a time to see if one causes the surge. If so, that’s the source of the leak.
3) Remove the air cleaner and observe the IAC valve movement with the engine running to verify IAC functionality.
It’s probably just my fatigued ear drums, but I don’t hear any surging in that video.
To verify if the RPM’s are actually changing, hook up an auxiliary tach or use a timing light with a tach function.
Surging at idle with no driveability concerns could be a vacuum leak; fuel supply, idle control or timing issue; or a torque converter/clutch pack that’s not fully releasing.
If you confirm a change in RPM, there’s a few checks to make with the engine at operating temperature and idling:
1) Disconnect the spark control circuit and check engine speed - if it stabilizes, there’s an issue with spark control, whether it’s distributor position or related sensors (cam and crank).
2) Unhook and cap/plug all external vacuum ports (brake booster, PCV, purge valve, MAP sensor, etc). If idle is stable, hook them up one at a time to see if one causes the surge. If so, that’s the source of the leak.
3) Remove the air cleaner and observe the IAC valve movement with the engine running to verify IAC functionality.
Last edited by Gumby22; Oct 30, 2023 at 7:24 PM.
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