C4 Surging
Thread Starter
Cruising
Joined: Sep 2004
Posts: 10
Likes: 0
C4 Surging
I recently purchased a 90 convert with 6 speed that was a theft recovery. We've fixed a lot of the problems, but when the car warms up it tends to surge while running at any steady RPM. We;ve changed all of the usual culprits (TPS, Plug Wires, etc.). It appears to run very lean while the surging occurs. Any ideas???
Team Owner
Joined: Aug 2000
Posts: 40,156
Likes: 45
From: San Pedro CA
gee... what about the IAC? What kinda RPM range are you talking about?
Perhaps a detailed list of what has been repaired, too. That will help us diagnose.
Do you have a service manual? If not, www.helminc.com is the source for the GM service manuals.
Perhaps a detailed list of what has been repaired, too. That will help us diagnose.
Do you have a service manual? If not, www.helminc.com is the source for the GM service manuals.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
I seem to remember reading that a warm idle surge from between 600 - 900 rpms shows something wrong with your IAC... but Bogus is right, what have you done so far? and exactly what RPM's are you surging to/from?
Drifting
Joined: May 2003
Posts: 1,979
Likes: 48
From: Lakeport CA
Big time lurk on this one... 
My '90 continues to surge in one scenario only:
Clutch in & moving at all. Soon as speedo drops to zero the idle drops to 800 & stays. Moving at all it constantly bounces from 800 to 1100. Doesn't really hurt anything but is annoying to roll up to a stop sign or something & the thing is going up & down in idle.
Will be interesting to see if the Formato chip I just received today from a fellow forum member makes any diff.
My '90 continues to surge in one scenario only:
Clutch in & moving at all. Soon as speedo drops to zero the idle drops to 800 & stays. Moving at all it constantly bounces from 800 to 1100. Doesn't really hurt anything but is annoying to roll up to a stop sign or something & the thing is going up & down in idle.
Will be interesting to see if the Formato chip I just received today from a fellow forum member makes any diff.
Racer
Joined: Aug 2004
Posts: 379
Likes: 170
From: Buffalo Grove IL
My '89 had a surged when warm, also had a bad idle in drive (A4) when sitting at a stoplight. New fuel injectors fixed the problem, but while reassembling everything I noticed the old throttle body-to-plenum gasket had a leak. How much of the surge was due to the injectors versus the leaky TB gasket I'll never know, though 
Perhaps do a resistance check on your fuel injectors to see if the coils are going bad? It's an easy check, only takes a couple minutes.
Ben
Perhaps do a resistance check on your fuel injectors to see if the coils are going bad? It's an easy check, only takes a couple minutes.
Ben
Pro
Joined: Jul 2004
Posts: 639
Likes: 0
From: 1985 L98, Auto. O'FALLON, IL
I had the exact same problem on my 85, Forum members suggested to check the resistance of the fuel injectors they should be about 17 ohms each. They also suggested disconnecting the o2 sensor and see if it stops surging when it is disconnected. Mine stopped surging and Idled better with it off. I replaced the 02 sensor for 20 bucks and it has run great since. Good luck and keep us posted as to what you find. 
Thread Starter
Cruising
Joined: Sep 2004
Posts: 10
Likes: 0
Since this was a theft recovery I'm not sure how much abuse it's sustained, but it apparently sat outside in the elements for a good deal of time. The surge/miss appears to happen at between 1K and 2K rpm, and is especially bad under a load - going uphill at steady speed.
So far we've adjusted valves, replaced plug wires, TPS Switch, coolant temp sensorDist cap & rotor, O2 sensor, and ran injector cleaner thru system. I do have a factory manual and just read about the IAC. Per the manual I disconnected the IAC and there was no change. However, when I reconnected it there has been some improvement. I'll see if it lasts and let you know.
Thanks
So far we've adjusted valves, replaced plug wires, TPS Switch, coolant temp sensorDist cap & rotor, O2 sensor, and ran injector cleaner thru system. I do have a factory manual and just read about the IAC. Per the manual I disconnected the IAC and there was no change. However, when I reconnected it there has been some improvement. I'll see if it lasts and let you know.
Thanks
Last edited by beachbumskip; Oct 7, 2004 at 01:16 PM.
Team Owner
Joined: Apr 2004
Posts: 36,040
Likes: 4
From: All great change begins at the dinner table Ronald Reagan
Originally Posted by beachbumskip
Since this was a theft recovery I'm not sure how much abuse it's sustained, but it apparently sat outside in the elements for a good deal of time. The surge/miss appears to happen at between 1K and 2K rpm, and is especially bad under a load - going uphill at steady speed.
So far we've adjusted valves, replaced plug wires, TPS Switch, coolant temp sensorDist cap & rotor, O2 sensor, and ran injector cleaner thru system. I do have a factory manual and just read about the IAC. Per the manual I disconnected the IAC and there was no change. However, when I reconnected it there has been some improvement. I'll see if it lasts and let you know.
Thanks
So far we've adjusted valves, replaced plug wires, TPS Switch, coolant temp sensorDist cap & rotor, O2 sensor, and ran injector cleaner thru system. I do have a factory manual and just read about the IAC. Per the manual I disconnected the IAC and there was no change. However, when I reconnected it there has been some improvement. I'll see if it lasts and let you know.
Thanks
Are the exhaust flanges tightened to the manifolds and don't leak at all?
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Le Mans Master
Joined: Feb 1999
Posts: 6,470
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From: levittown pa. usa Even a bad day with my `Vette, is better than a good day at work
St. Jude Donor '10
My `87 used to surge up & down at idle. I took the throttle body off & cleaned it with spray carb cleaner. No more surge
Drifting
Joined: May 2003
Posts: 1,979
Likes: 48
From: Lakeport CA
Well... haven't heard anything really new here. Drove to town today for the first time w/ my "new to me" chip & no change on idle hunt. My injector resistance is fine, checked it a few weeks ago. TPS range was good last time I checked it (non adjustable year). TB is totally clean as I cleaned it while the engine was all apart last month. Even took out the IAC & made sure it was clean & did a reset & confirmed base idle. Haven't priced an IAC but replacing it is about the only thing I haven't done. :-(
Holy C... I just checked the price on one... GM Parts Direct is even $52.00 (plus shipping of course), Napa or Kragens etc is like $75?? WTF
Anyone got a spare I can do some testing with? I'll buy it if its what I need but I can't see throwing $ at the problem w/ out knowing for sure.
Holy C... I just checked the price on one... GM Parts Direct is even $52.00 (plus shipping of course), Napa or Kragens etc is like $75?? WTF
Anyone got a spare I can do some testing with? I'll buy it if its what I need but I can't see throwing $ at the problem w/ out knowing for sure.
Last edited by Trog; Oct 10, 2004 at 02:23 AM.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
Originally Posted by Trog
Holy C... I just checked the price on one... GM Parts Direct is even $52.00 (plus shipping of course), Napa or Kragens etc is like $75?? WTF
Anyone got a spare I can do some testing with? I'll buy it if its what I need but I can't see throwing $ at the problem w/ out knowing for sure.
Anyone got a spare I can do some testing with? I'll buy it if its what I need but I can't see throwing $ at the problem w/ out knowing for sure.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
Originally Posted by Trog
What was the part number on yours? I just checked Autozone.com and the part was $58.99??
http://www1.autozone.com/servlet/UiB...+CONTROL+VALVE