Chevy TBI idle problems (1 Viewer)

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Jul 15, 2005
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Location
High Desert, SoCal
Recently rebuilt the 454 in my 1990 TBI Burb. I am having a terrible time with getting it to idle smooth. The engine has a little over 500 miles on it. I have spent countless hours going over the engine looking for vacuum leaks and believe every thing is OK on that front. I have replaced the engine temp sensor, the connections to the coil, the coil, cap, rotor, plug wires, map, checked the tps, reset the base idle, checked voltage values at the tps connection, map connection etc. The iac was replaced about 4000 mles ago. The throttle body was re-bushed about 4000 miles ago. The egr seems fine. I took it off and cleaned it. Checked it with a vacuum pump both on and off the truck. The engine is all stock except for the Edelbrock manifold. The truck idles like crap in park, but idles smooth in drive. It seems worse cold then after it warms up.

I took it to get smogged (Ca) and it failed (high NO). The tech said it would pass easily with a new cat. The old one was just 2 years old. About the only parts that aren't new is the O2 sensor (replaced 2 years ago), the iac and the egr vacuum solenoid.

I am about ready to pull my hair out with this one.
Thanks Brad
 
you did everything I know of, a bad o2 sensor could cause that.....my egr did the same thing, have you actually plugged the hose coming off it to see if it helped?
 
Yeah, tried that. didn't change anything. I even replaced the egr with a new one, thinking that maybe the old one wasn't closing all the way. Still idled like crap. Put the old one back on as it was an AC. What is weird is that the truck idles good in drive, but in park it sounds terrible. What would cause that? :bang: Can't I test the O2 sensor with an ohm meter? I think I remember reading it somewhere. Anyone know what it should be reading cold/ warm ?

Thanks Brad
 
The truck has TBI, not a carb. I have been all over the intake looking for leaks, because that was what I thought at first. I have used propane, carb cleaner, and wd40 to try to locate leaks. I found one on the pb port and repaired it. I replaced the TBI gasket again, just to be sure. The only area I can't get to is the rear of the manifold, behind the dizzy. I don't believe there is a leak, I tested with a vacuum gauge and the vacuum holds steady.
Thanks Brad
 
O2 sensor won't have any effect when cold, its signal is ignored.

Are you sure the rebuild is 100%? It wouldn't hurt to run a compression check to be positive.

High NoX can also indicate a lean condition, not necessarily a vacuum leak, but something else that could cause it to run lean. Do you know anyone with a scanner who could look at the datastream for anything out of the ordinary?
 
I am pretty sure the rebuild is good. The folks that did the job (Edding's Engine in San Fernando) have a great reputation for building sound engines. I could check it just to be sure. It is all stock with the exception of the intake manifold. I didn't think the O2 could be bad, but pulled it anyway and tested it. It was a little slow in generating voltage and the changing of voltage was not as quick as it should have been. I went ahead and replaced it. I checked the EGR solenoid today and it wouldn't hold vacuum. The filter was shot too. I replaced the solenoid.

I really don't have access to a scanner.

When I changed the plugs a couple of days ago I noticed the plug deposits seemed strange. I switched out some Bosch copper for AC. On almost all the plugs the deposits were only on one side. The other side was clean (like new). I have never seen plugs look like that.

Could an injector problem cause the lean condition? It is really strange that the miss disappears under load.

Thanks Brad
 
Get a timing light out and point it at the injectors. Compare the two patterns. Are they the same?

I would reccomend an aldl cable and find a laptop or old computer and run some log files. There is a couple of good forums that would probably be willing to tell you if they see anything.
 
Thanks I'll check it out with a timing light. Is any one cylinder better then the other to use the light on?
Thanks Brad
 
Just got done with the timing light test and they both look the same to me. I did notice that the spray didn't look as fine as I thought it would. The spray looked rather coarse. I am going to search for a photo of tbi spray and see what it is supposed to look like so I have a reference.
Thanks Brad
 
Haven't found any decent pictures. Anyone have a link or picture you can send me or post? I examined the injectors again with just a flash light. Injector spray doesn't look exactly the same, but both sides are just about the same size. The spray droplets look rather large on both. I drove the truck around for a bit and the surge all but goes away when idling in drive. You can just barely feel it if you are concentrating and looking for it. I am not yet totally convinced it is the injectors, but at this point any direction is better then none. :bang: Has anyone here used Witch Hunter Performance to have their injectors cleaned and checked? Their website is here.
Unfortunately there doesn't seem to be anyone locally that offers this kind of service.


Thanks Brad
 
Brad,

I found this website by googling "1990 Chevrolet Suburban surges off idle". I Also have a 1990 suburban that idles irradically and surges from a stop. I followed what you checked and did the same. I found another website that had a forum that the final problem ended up being the EGR Valve. I checked my EGR Valve and it was working fine. I removed it and there were hardly any deposits on it. At this point in my diagnosis I had checked EVERYTHING possible. Since this other site insisted there problem was the EGR, I called my local Napa and found the the valve is $120.00. Right now I'm a striking Union member of the Boeing Company, so I did not want to waste the money unless I knew this was the problem. I fabricated a EGR block off plate, installed it, and it made NO DIFFERENCE.

I found the problem by dumb luck!! I removed the IAC after performing the 2 resistance checks (checked out OK). It turned out that the tip of the air valve had a set of wear marks in it that were sticking in the throttle body casting. I push the valve to the "OPEN" position, cleaned it and reinstalled it. When I tightened it up, I turn it about an 1/4 turn past it's original location so the tip of the valve had a new area to seat against. I fired it up, problem GONE!!!! It drives great now.

I don't know if this will help you, but spent two days trying to figure out what was wrong.

Good Luck
Scott in Seattle
 
Thanks for the info.

I have been over and over my engine, so may times, I think I could disassemble the top half in my sleep. The other day I got out the propane torch again to look for vacuum leaks (5th time). I noticed that there was a slight increase in rpms when the propane was applied on the right side close to the IAC. I thought it was either the IAC or perhaps the manifold. I re-torqued all the manifold bolts and removed the IAC . It like yours had a couple of scuff marks. I went to the P&P and found another IAC. It looked perfect. I put it in and started the truck. I thought I had solved the problem. The engine sounded good at fast idle, then as the engine slowed the surge returned. I put the truck in drive and the surge disappeared. I went back over it again with the propane and could find no leaks.

I did catch something that I hadn't seen before. I noticed that the gas as it is sprayed from the injector hits the bottom half of the injector support tower and then drips into the throttle body. Maybe that is what is causing the problem? Anyone know if that is a problem or is it normal?
I don't want to send the injectors out if this is normal.

Thanks Brad
 
Curious if you ever got this idle problem nailed down... I have 95 K2500 that with a 454 (Gen V TBI) that i just rebuilt (everything back to stock except bored over .030"). I've had difficulty controling my idle every since i've owned the vehicle, even before the rebuild; and i've tested/replaced about everything that you've mentioned. I also have inconsistant plug symptoms... Half of them are carboned very badly (some of them have deposits on only one side...), and the other plugs look as though their associated cylinders are running too lean... Its also been running a little hot, and i think im loosing a little coolant (internally.... uh oh!)..

I just finished reading some info at GM TBI Swap that mentions a park/neutral switch that is supposed to adjust idle when in park/neutral... I was unaware of such a switch, but sounds like it could be related to your problem....

Please let me know what you find/found out

Thanks,

Someguy
 
I still haven't fixed the problem. The truck runs pretty good in drive when it is warmed up. Since the weather has gotten colder, the truck idles worse when first started. It surges badly before it is warm. It passed Ca smog easily and I have been driving it. I lost my job, so I haven't had any extra money to get the fuel injectors cleaned or take it to a shop.
I don't know if the switch could be the problem. The truck idle steps up and down as it is put into park/drive. Perhaps I have a ground wire problem. I will check that out as soon as the weather clears. It's snowing now. Thanks for the suggestion.

Are you using the stock intake? The heater that goes between the TBI and intake sometimes leak. My old one did. where the hose was crimped on. Perhaps you are losing fluid there. Unless you are seeing oil in the water or water in the oil, you are probably OK. Another sign is water and white smoke coming from the tailpipe. If you think it's a head gasket problem, I would recheck the head torque before repalcing the gasket. Sometime retorqing the bolts will seal a minor leak. Do not over torque. Did you have the heads surfaced?

I noticed my plugs had deposits on only one side too. Not sure what causes that. If and when I get some money together I am going to have Witchhunter do my injectors. I think they could be part of the problem I am having. Of course the wiring on my truck is 19 years old, so there could be a problem with it.

Sorry I don't have a solution. After all the time and moiney I have spent it would be nice to have an answer to share.

Best regards Brad
 
Does that engine have the platic vaccume lines on the EGR solonoid? IF so you might as well through those in the garbage and use "old-school" vaccume hose. I know my former 92K1500 with 5.7 was very senstitive to base timing. It had to be at zero degrees.....nothing less, nothing more.

Came across somthing different the other day...friend had cylinder heads rebuilt on 5.0 vortec...never could get the idle right....turns out intake gasket was leaking internally (to the lifter valley). I assume you have no trouble codes?
 
Ran across this old post of mine and figured I should wrap it up. After 2 years and having the burb at 2 different shops with no result to the surging idle, I stumbled upon the answer at a Ford site. Surging idle can be caused by a number of different problems. IAC, EGR, EGR controller, MAP, temp sensor, vacuum leaks, O2 etc. But hardly anyone references the butterfly on the TBI unit not being adjusted right. It seems if the butterfly is closed and not open enough to begin with, the ecm will never find the correct idle. It will continue to hunt and cause the surge. While I first found the explanation at a Ford site, a bit of research found this handy set of instructions at ThirdGen.org.

The link is: IAC and TPS Adjustment - ThirdGen.org

After resetting the base idle, the truck doesn't surge. It seems the butterfly on the burb was too far closed. So before replacing $$$$$ of parts make sure the TBI base idle is adjusted.
 

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