1985 Idle surging after warmup - CorvetteForum - Chevrolet Corvette Forum Discussion



C4 Tech/Performance
L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

1985 Idle surging after warmup

Reply
 
 
 
Thread Tools Search this Thread
Old 07-16-2017, 01:57 AM   #1
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default 1985 Idle surging after warmup

I have read through several threads and can't seem to find a answer for a familiar problem. I have an 85, which I bought a month ago. The car has 58,000 miles on it and has been running like a champ. A few days ago I decided to upgrade the stereo system and removed the stock stereo. The following day, the car started surging (jumping up and down from 400-900 rpm) at idle after it reached operating temperature. sometimes it will die after a bit but not always. I did replace the IAC and cleaned the TB. TPS is at .54 and cleaned the MAF. Couple of interesting things: 1) if I let the car sit overnight and start it the next day, it will not surge on the first start, even when it hits operating temp. Once I shut it off and restart, it will start the surging after about 15 seconds or so. Also if I turn on the A/C or put the car in gear the idle will smooth out. If the AC is on and in park as long as the compressor is engaged the idle will be smooth, when the compressor dis-engages it will surge again. When I put the car in gear and leave it with the brake on idle again smooths out, put in park and it is fine until I give it a bit of throttle, once released it will surge again. No codes at all. Any help it greatly appreciated. I'm thinking a vacuum leak or maybe a bad egr. I will try to figure out a way to do a smoke test tomorrow.
OsosBob is offline   Reply With Quote
Old 07-16-2017, 01:19 PM   #2
billschroeder5842
ZenVette Master level VII
Support Corvetteforum!
 
billschroeder5842's Avatar
 
Member Since: Mar 2016
Location: Southlake, TX
Posts: 1,676
Thanked 160 Times in 146 Posts
Default

If you have made all the adjustments that you mentioned and adjusted all the setting, I'd guess that you have a vacuum leak.


Retighten all the manifold bolts and look at all the hoses coming off the engine as a small air leak can cause that surging you are experiencing.


If you find a cracked hose, replace it or often times you can cut a 1/4 inch from the end of the hose and re attach.
billschroeder5842 is offline   Reply With Quote
Old 07-16-2017, 01:25 PM   #3
whalepirot
CF Senior Member
 
whalepirot's Avatar
 
Member Since: Jan 2008
Posts: 1,436
Thanked 28 Times in 28 Posts
Default

Quote:
Originally Posted by billschroeder5842 View Post
I'd guess that you have a vacuum leak.
Start with a vacuum gauge and look up how to read one. They can be an amazing tool. Hopefully, you didn't unintentionally disconnect a vac line in the dash.

The variance occurs in closed loop, when all the sensor input drives the ECM control and vacuum level is an input for how hard the engine is pulling and controls many pneumatic 'motors'.
whalepirot is offline   Reply With Quote
Old 07-16-2017, 02:13 PM   #4
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

can you just clarify that it was not doing any of this before a radio change?

when you installed your radio was the battery disconnected?
Given all the conditions that you can obtain a smooth idol, I'm leaning towards something electrical was disturbed during the radio install.
VikingTrad3r is offline   Reply With Quote
Old 07-16-2017, 02:24 PM   #5
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Quote:
Originally Posted by VikingTrad3r View Post
can you just clarify that it was not doing any of this before a radio change?

when you installed your radio was the battery disconnected?
Given all the conditions that you can obtain a smooth idol, I'm leaning towards something electrical was disturbed during the radio install.
The car was running just fine prior to the removal of the radio and yes I did disconnect the battery. I purchased this car from a guy who rarely drove it. I have noticed most of the vacuum hoses are dry (black rubber comes off with any kind of touching) but I have not seen any cracks but I know they are in poor shape. I was thinking about taking apart the plenum and just replacing all vac lines, cheap enough. Still need to do a smoke test. Radio is still out of the car, I have not installed the new one yet.

Last edited by OsosBob; 07-16-2017 at 02:24 PM.
OsosBob is offline   Reply With Quote
Old 07-16-2017, 02:40 PM   #6
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

myself, I am a non-smoker. But I sucked it up and went and bought two cigars. I pulled the accordion style intake tube off of the throttlebody and use masking tape to mask the throttlebody off thereby creating a seal there. Then I removed the large vacuum line off of the brake booster, carefully, so as not to ruin the valve there. And I began to blow a heap of smoke hard and fast into that hose. The smoke travels through the brake booster vacuum line into the Plenum and then throughout the engine.
My wife, who is also a non-smoker, sucked it up and came into the garage with her iPhone light in the flashlight and began looking for where the smoke tendrils were coming out.This is a two person job to find the smoke. It came out best when I was blowing hard.

for me, on my 85, it was the EGR vacuum tube on the driver side fitting on to the nipple off of the intake manifold. The manifold side of that junction was leaking badly. After discovering the smoke. I got a simple hose clamp and with a series of long extensions was able to screw it tight around the hose on that nipple. Problem solved for me.

grab a couple cigars and an assistant and you should find your leak if you have one.

personally, I doubt that it is a vacuum leak at this point, if it is, it's a very small one. The reason I say that is that when you put the vehicle in park and the idol returns to normal, that changes nothing about the vacuum leak. If you had a vacuum leak you would still have a vacuum leak when the vehicle was in park.

mini vacuum leaks will become be nine at higher rpm's because the unmetered air is a much smaller percentage of the air passing through the system. But when you are idling, either in park or neutral, the unmetered air is a much higher percentage of the amount of air required. This is also the highest vacuum State in the engine.

it would be smart to do the vacuum test because it is easy cheap and fast. Let's rule that one out or if you're lucky, perhaps that is your problem.

you mentioned pulling the runners and plant them, I would strongly advise against that at the present time. If you see the cables are weak and brittle, you will most assuredly cause many problems in the process of pulling it apart. I say this only if you intend or desire to drive it immediately. If the car is a summer project have at it!
VikingTrad3r is offline   Reply With Quote
Old 07-16-2017, 04:15 PM   #7
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Update: Did a smoke test, got to love a fog machine; it pays to be a Halloween nut, I had a very small amount of smoke exiting the throttle body linkage and a small amount of smoke from a silver tube on the drivers side. That tube had a rubber hose with no clamp, I put on a clamp and smoke test again. No more smoke from the drivers side. A quick test at idle and once again it surges. One thing I also notice is it seems like there is a bit more water dripping from the tail pipes than it should. Also if I rev the engine up sometimes it will find it's happy place and idle just fine but again the minute I apply a small amount of throttle and the idle settles down it surges. I'm going to put the stereo back together and then consider taking apart the plenum. I'm also wondering if the fact the car sat without running for a very long period of time and then me buying the car running it almost daily for a month caused some gaskets to crack. If that happened it would explain a small amount of water in the exhaust and possibly the idle issue. Again any ideas is appreciated.
OsosBob is offline   Reply With Quote
Old 07-16-2017, 04:43 PM   #8
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

are you thinking the intake manifold /top of head gasket?
VikingTrad3r is offline   Reply With Quote
Old 07-16-2017, 11:39 PM   #9
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Quote:
Originally Posted by VikingTrad3r View Post
are you thinking the intake manifold /top of head gasket?
I'm thinking more like intake manifold gasket(s). I'm hoping not a head gasket but that is a possibility. I'm not having any temp issues. I hate to rip into the engine as I really wanted to enjoy the car this summer but since I can't smog it here in Cali like it is it may be sitting awhile. I guess before I pull apart the intake I will do a compression test.
OsosBob is offline   Reply With Quote
Old 07-17-2017, 02:01 AM   #10
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

yes that is the gasket i was referring to

my 85 had a leaking intake manifold gasket. it was likely just the loose screws.
VikingTrad3r is offline   Reply With Quote
Old 07-17-2017, 10:30 AM   #11
whalepirot
CF Senior Member
 
whalepirot's Avatar
 
Member Since: Jan 2008
Posts: 1,436
Thanked 28 Times in 28 Posts
Default

A small amount of water in the exhaust may just be condensation, until it warms up.

Switrching on the A/C bumps up the idle, to counter the power draw, jus tlike adding throttle. That, added to what viking said, might cover up a small vacuum leak.

IF you do a compression test, 'read' the plugs. I have a piece of wood, with 8 holes drilled, each having a cylinder number, to aid that process. I pull each plug then invert it into a hole for comparison and analysis of the combustion leftovers. My antique Motor manual has a great back inside cover which abets that analysis.

Last edited by whalepirot; 07-17-2017 at 10:31 AM.
whalepirot is offline   Reply With Quote
Old 07-17-2017, 10:53 AM   #12
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

regarding what whale said, my 85 had a blown head gasket at 7 and 6 and it was easy to see on the plugs.

i used a piece of white paper.

when i was at your exact stage with my 85, john marker made a comment saying that if i re-sealed the entire engine down to the headgaskets, i would have a dramatically better engine and he was so right. this takes car of things like injector orings as well

there is also an updated chip released by gm at the very end of 85/early86 that helps a lot for the 85's. Not sure how far down the rabbit hole you want to go.

if you want to read a saga of my 85 with surging / gasping! idle sometimes konking out, here you go:

https://www.corvetteforum.com/forums/c4-tech-performance/3735399-seeping-intake-bolts-1985-affect-idle.html

Last edited by VikingTrad3r; 07-17-2017 at 10:54 AM.
VikingTrad3r is offline   Reply With Quote
Old 07-17-2017, 11:03 AM   #13
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

while i think about it and this may have already been said but if your runs fine in open loop vs closed loop then chances are its a sensor / electrical lroblem.
VikingTrad3r is offline   Reply With Quote
Old 07-17-2017, 12:02 PM   #14
383vett
CF Senior Member
 
383vett's Avatar
 
Member Since: Apr 2003
Location: moraga ca
Posts: 13,880
Thanked 225 Times in 170 Posts
Default

Still trying to find out how disconnecting the battery and removing a radio could cause this problem. I can't believe a blown head gasket or an instantaneous vacuum leak could have happened with the motor off and the car sitting in the garage. I would back up and look more toward an electrical problem. Could the ignition have been on when the battery was reconnected causing a surge to the ECU? Is it possible the iac's need to be reset?
383vett is online now   Reply With Quote
Old 07-17-2017, 12:05 PM   #15
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 3,401
Thanked 239 Times in 223 Posts
Default

when battery was disconnected was it the negative that was unplugged first?
VikingTrad3r is offline   Reply With Quote
Old 07-18-2017, 10:06 AM   #16
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Quote:
Originally Posted by VikingTrad3r View Post
regarding what whale said, my 85 had a blown head gasket at 7 and 6 and it was easy to see on the plugs.

i used a piece of white paper.

when i was at your exact stage with my 85, john marker made a comment saying that if i re-sealed the entire engine down to the headgaskets, i would have a dramatically better engine and he was so right. this takes car of things like injector orings as well

there is also an updated chip released by gm at the very end of 85/early86 that helps a lot for the 85's. Not sure how far down the rabbit hole you want to go.

if you want to read a saga of my 85 with surging / gasping! idle sometimes konking out, here you go:

https://www.corvetteforum.com/forums...fect-idle.html
VikingTrad3r, thanks for the info and link to your thread, I think my issue is very similar to yours without the tranny issue. I put in a new O2 sensor this weekend since they are cheap and no change. I ordered a scanner over the weekend and will wait till it gets here so I can get more data. I have a sneaky suspicion it is a gasket issue. Looking over the repair records the previous owner has it would appear they have changed the intake gasket and replaced injectors, plugs, wires and basic tune up. 8,000 miles ago the car had a compression check and showed cylinder #8 at 140 while the others were 160-175. My gut feeling tells me I'm going to pull the engine and do a complete go through. May be overkill but it just may be a better option. By the way I was trying to pull the burnoff relay and have a look at it but could not seem to figure out how to take of the passenger lower carpeted panel. Can anyone help me out with this?
OsosBob is offline   Reply With Quote
Old 07-18-2017, 10:51 PM   #17
TBSS/85vette
CF Senior Member
 
TBSS/85vette's Avatar
 
Member Since: Dec 2012
Location: Rockford Illinois
Posts: 264
Thanks: 0
Thanked 5 Times in 5 Posts
Default

My 1985 idled exactly as yours did. Do you have a picture of the silver tube that you described? Does it go into the side of the plenum on the drivers side? If so check that soft rubber gasket it inserts into. Mine got real soft and wouldn't seal tightly. I put a small zip tie there to seal a vacuum leak there and my idle has been perfect ever since. I don't think the stereo has anything to with it. Condensation out the exhaust on a cold engine is normal. When you drive it and the engine heats up it stops.
TBSS/85vette is offline   Reply With Quote
Old 07-19-2017, 12:58 AM   #18
OsosBob
Junior Member
Thread Starter
 
Member Since: Jul 2017
Posts: 6
Thanks: 0
Thanked 0 Times in 0 Posts
Default

Quote:
Originally Posted by TBSS/85vette View Post
My 1985 idled exactly as yours did. Do you have a picture of the silver tube that you described? Does it go into the side of the plenum on the drivers side? If so check that soft rubber gasket it inserts into. Mine got real soft and wouldn't seal tightly. I put a small zip tie there to seal a vacuum leak there and my idle has been perfect ever since. I don't think the stereo has anything to with it. Condensation out the exhaust on a cold engine is normal. When you drive it and the engine heats up it stops.
That is the same silver tube, I was able to put a clamp on it but no change. I doubt the stereo had anything to do with it but it was the only change prior to the issue starting. By the way looks like your Corvette and mine are long lost cousins
OsosBob is offline   Reply With Quote
Go Back   CorvetteForum - Chevrolet Corvette Forum Discussion >
Reload this Page
  • 1985 Idle surging after warmup
  •  
     
    Reply


    Thread Tools Search this Thread
    Search this Thread:

    Click for Advanced Search

    Posting Rules
    You may not post new threads
    You may not post replies
    You may not post attachments
    You may not edit your posts

    BB code is On
    Smilies are On
    [IMG] code is On
    HTML code is Off
    Trackbacks are Off
    Pingbacks are Off
    Refbacks are Off

    Forum Jump


    All times are GMT -4. The time now is 02:52 PM.


     
    • Ask a Question
      Get answers from community experts
    Question Title:
    Description:
    Your question will be posted in: