L98 idle surge
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 47
Likes: 0
From: Latte Heights Guam
L98 idle surge
Ok guys! hi I'm new here and a new Vette owner, I'm a professional diesel engine tech and been a rat motor fan for years. I'm hoping you can help me figure out this problem.
when i first start engine cold it idles fine and smooth at about 7-800 RPM, but once the engine warms up the idles surges from like 600-1100 but not methodically, more like it tries to die then the computer gives it more fuel.
i have done all the basic stuff, spark plugs, cap and rotor, i even dinked around with the EGR crap as that is the direction the service manual directed me. however nothing i do changes any of the characteristics of the problem.
has anyone experienced this before? can you give me any ideas of where to go from here.
thanks!
when i first start engine cold it idles fine and smooth at about 7-800 RPM, but once the engine warms up the idles surges from like 600-1100 but not methodically, more like it tries to die then the computer gives it more fuel.
i have done all the basic stuff, spark plugs, cap and rotor, i even dinked around with the EGR crap as that is the direction the service manual directed me. however nothing i do changes any of the characteristics of the problem.
has anyone experienced this before? can you give me any ideas of where to go from here.
thanks!
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Share engine info, year, model, mods, etc. Any codes?
Seems like a Closed Loop issue at first glance: IAC, TPS, 02, Coolant Temp Sensor, MAF (?), EGR, Vacuum Leak, etc.
More details needed.
Jake
West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!
Seems like a Closed Loop issue at first glance: IAC, TPS, 02, Coolant Temp Sensor, MAF (?), EGR, Vacuum Leak, etc.
More details needed.
Jake
West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!
Team Owner
Joined: Jan 2009
Posts: 73,568
Likes: 33
From: Somewhere in Mo
St. Jude Donor '09-'10, '14
Share engine info, year, model, mods, etc. Any codes?
Seems like a Closed Loop issue at first glance: IAC, TPS, 02, Coolant Temp Sensor, MAF (?), EGR, Vacuum Leak, etc.
More details needed.
Jake
West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!
Seems like a Closed Loop issue at first glance: IAC, TPS, 02, Coolant Temp Sensor, MAF (?), EGR, Vacuum Leak, etc.
More details needed.
Jake
West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!
I was just reading about this issue today.
Last edited by RetiredSFC 97; Dec 29, 2009 at 05:26 PM.
Melting Slicks
Joined: Aug 2003
Posts: 3,395
Likes: 393
From: Lakeville MI
Idle surge can be difficult to solve.
If everything is working properly, it is likely due to a misadjusted tps sensor voltage and/or throttle minimum air adjustment or a result of unmetered air (vacuum leaks).
Accepted range for tps sensor at idle is 0.54 to approx. 0.64 volts.
Note: The tps load axis calculation considers 0.69 volts as zero % tps, so be sure that your setting is at least below 0.69 volts.
Your cold idle 700-800 rpm sounds a bit low, but this is dependent upon the coolant temperature.
Here's a typical 86 auto idle rpm vs coolant temp table:
Idle RPM vs Coolant Temperature
Deg F RPM
(null)
304 625.00
284 625.00
262 625.00
241 625.00
219 625.00
198 625.00
176 625.00
154 625.00
132 625.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
3 1050.00
-18 1050.00
-40 1050.00
Add a 50 rpm offset for park or neutral and you should expect a cold idle of around 1,000 rpm, which gradually drops to 675 in park/neutral as it warms up.
If the engine is starved for air, as the rpms fall, the IAC will open (without much effect), the calculated load will increase resulting in mixture enrichment (if still in open loop) and will also affect the spark advance as a function of rpm.
As revs continue to drop, stall saver spark advance will eventually kick in in an attempt to revive a stalling engine.
You have a very dynamic situation where the rpm, IAC position, load, target AFR and spark advance are all changing rapidly. This doesn't promote a stable idle and can easily result in surge or stalling.
For the IAC to have a chance to control the idle by itself it must have a reasonable adjustment range and the spark advance and mixture must be relatively stable.
Follow the min air adjustment procedure (do a search on this topic) to ensure the tps and IAC are in their proper ranges first.
If problems persist, then you can dig deeper as required. Check for vacuum leaks, verify fuel pressure, coolant temp and MAF signals, proper IAC function etc.
If everything is working properly, it is likely due to a misadjusted tps sensor voltage and/or throttle minimum air adjustment or a result of unmetered air (vacuum leaks).
Accepted range for tps sensor at idle is 0.54 to approx. 0.64 volts.
Note: The tps load axis calculation considers 0.69 volts as zero % tps, so be sure that your setting is at least below 0.69 volts.
Your cold idle 700-800 rpm sounds a bit low, but this is dependent upon the coolant temperature.
Here's a typical 86 auto idle rpm vs coolant temp table:
Idle RPM vs Coolant Temperature
Deg F RPM
(null)
304 625.00
284 625.00
262 625.00
241 625.00
219 625.00
198 625.00
176 625.00
154 625.00
132 625.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
3 1050.00
-18 1050.00
-40 1050.00
Add a 50 rpm offset for park or neutral and you should expect a cold idle of around 1,000 rpm, which gradually drops to 675 in park/neutral as it warms up.
If the engine is starved for air, as the rpms fall, the IAC will open (without much effect), the calculated load will increase resulting in mixture enrichment (if still in open loop) and will also affect the spark advance as a function of rpm.
As revs continue to drop, stall saver spark advance will eventually kick in in an attempt to revive a stalling engine.
You have a very dynamic situation where the rpm, IAC position, load, target AFR and spark advance are all changing rapidly. This doesn't promote a stable idle and can easily result in surge or stalling.
For the IAC to have a chance to control the idle by itself it must have a reasonable adjustment range and the spark advance and mixture must be relatively stable.
Follow the min air adjustment procedure (do a search on this topic) to ensure the tps and IAC are in their proper ranges first.
If problems persist, then you can dig deeper as required. Check for vacuum leaks, verify fuel pressure, coolant temp and MAF signals, proper IAC function etc.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
Make sure all the initial set-ups are correct as Tequilaboy has shown and then see if you still have the problem. If you do my guess is the O2 sensor is not reading correctly or is unable to correct for a lean fuel mixture due to a mechanical issue. A surge at idle is ussually caused by a lean fuel mixture and since your car does not surge until it warms up and goes into closed loop operation, my thought is that it is lean. If you can get a scanner check to see what your BLM's are at idle when warm, they should be near 128. If they are higher then 128 (example 145) the idle mixture is lean and the car is attempting to add fuel to correct the problem. If the O2 has failed it could still be reading 128 or close and actually be reading incorrectly and may need replacement (This is a last ditch effort once you have verified everything else is correct) .
Instructor
Joined: Feb 2008
Posts: 177
Likes: 5
From: Toronto Ontario
I had exactly the same problem and went down the same route looking at ignition, EGR, TPS ,O2 sensor and so on. The problem turned out to be bad injectors.
Use an OHM meter to check the resistance of each injector both hot and cold. They should be in the 16 ohm range. Bad injectors change the air/fuel mix and can cause the engine to do weird things like surging.
Do you have any Codes?
Use an OHM meter to check the resistance of each injector both hot and cold. They should be in the 16 ohm range. Bad injectors change the air/fuel mix and can cause the engine to do weird things like surging.
Do you have any Codes?
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 47
Likes: 0
From: Latte Heights Guam
thanks guys for all the tips, I will work on your suggestions and get back to you!
the car specs as follows, 1986 C4 L98 .30 over pistons, magnaflow catbacks. the rest is stock.
thanks again
the car specs as follows, 1986 C4 L98 .30 over pistons, magnaflow catbacks. the rest is stock.
thanks again
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Brett Foote
Team Owner
Joined: Jul 2004
Posts: 28,432
Likes: 3,285
From: Hartford WI
I'd take the entire throttle body apart and have it cleaned along with the IAC housing. Use a scantool to reset the idle rates to about 20 counts and see that the TPS is smooth when you gently step on the pedal. Any dead spots and you need a new TPS.
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 47
Likes: 0
From: Latte Heights Guam
Idle surge can be difficult to solve.
If everything is working properly, it is likely due to a misadjusted tps sensor voltage and/or throttle minimum air adjustment or a result of unmetered air (vacuum leaks).
Accepted range for tps sensor at idle is 0.54 to approx. 0.64 volts.
Note: The tps load axis calculation considers 0.69 volts as zero % tps, so be sure that your setting is at least below 0.69 volts.
Your cold idle 700-800 rpm sounds a bit low, but this is dependent upon the coolant temperature.
Here's a typical 86 auto idle rpm vs coolant temp table:
Idle RPM vs Coolant Temperature
Deg F RPM
(null)
304 625.00
284 625.00
262 625.00
241 625.00
219 625.00
198 625.00
176 625.00
154 625.00
132 625.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
3 1050.00
-18 1050.00
-40 1050.00
Add a 50 rpm offset for park or neutral and you should expect a cold idle of around 1,000 rpm, which gradually drops to 675 in park/neutral as it warms up.
If the engine is starved for air, as the rpms fall, the IAC will open (without much effect), the calculated load will increase resulting in mixture enrichment (if still in open loop) and will also affect the spark advance as a function of rpm.
As revs continue to drop, stall saver spark advance will eventually kick in in an attempt to revive a stalling engine.
You have a very dynamic situation where the rpm, IAC position, load, target AFR and spark advance are all changing rapidly. This doesn't promote a stable idle and can easily result in surge or stalling.
For the IAC to have a chance to control the idle by itself it must have a reasonable adjustment range and the spark advance and mixture must be relatively stable.
Follow the min air adjustment procedure (do a search on this topic) to ensure the tps and IAC are in their proper ranges first.
If problems persist, then you can dig deeper as required. Check for vacuum leaks, verify fuel pressure, coolant temp and MAF signals, proper IAC function etc.
If everything is working properly, it is likely due to a misadjusted tps sensor voltage and/or throttle minimum air adjustment or a result of unmetered air (vacuum leaks).
Accepted range for tps sensor at idle is 0.54 to approx. 0.64 volts.
Note: The tps load axis calculation considers 0.69 volts as zero % tps, so be sure that your setting is at least below 0.69 volts.
Your cold idle 700-800 rpm sounds a bit low, but this is dependent upon the coolant temperature.
Here's a typical 86 auto idle rpm vs coolant temp table:
Idle RPM vs Coolant Temperature
Deg F RPM
(null)
304 625.00
284 625.00
262 625.00
241 625.00
219 625.00
198 625.00
176 625.00
154 625.00
132 625.00
111 750.00
90 900.00
68 950.00
46 1050.00
24 1050.00
3 1050.00
-18 1050.00
-40 1050.00
Add a 50 rpm offset for park or neutral and you should expect a cold idle of around 1,000 rpm, which gradually drops to 675 in park/neutral as it warms up.
If the engine is starved for air, as the rpms fall, the IAC will open (without much effect), the calculated load will increase resulting in mixture enrichment (if still in open loop) and will also affect the spark advance as a function of rpm.
As revs continue to drop, stall saver spark advance will eventually kick in in an attempt to revive a stalling engine.
You have a very dynamic situation where the rpm, IAC position, load, target AFR and spark advance are all changing rapidly. This doesn't promote a stable idle and can easily result in surge or stalling.
For the IAC to have a chance to control the idle by itself it must have a reasonable adjustment range and the spark advance and mixture must be relatively stable.
Follow the min air adjustment procedure (do a search on this topic) to ensure the tps and IAC are in their proper ranges first.
If problems persist, then you can dig deeper as required. Check for vacuum leaks, verify fuel pressure, coolant temp and MAF signals, proper IAC function etc.
Darrell
Last edited by catmech; Jan 1, 2010 at 04:17 PM. Reason: repair
