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Stumbling idle....NEED HELP PLEASE!!!!!!

K

K5DAVID

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k, well i have 88 blazer with the TBI 350ci. i have owned the truck for 4 months. i just recently discovered that there was never a 02 sensor on it and my engine light was always fading in and out while driving. previous owner installed long tube headers but never installed bung and 02 sensor. so yesterday i bought a bung, drilled a hole and welded it in and installed a brand new bosch 02 sensor (direct replacement). but before doing this my truck always had a stumbling idle kinda like a light misfire in a cylinder and would sometimes die when putting it in drive. well after installing the 02 sensor, the truck does run much better when "DRIVING" but now when im sitting at a red light idling, the truck has a very bad stumble as if i am turning the key off and then back on suddenly before it kills the engine. i also replaced all the plugs with ac delcos yesterday. the truck runs great while driving it, it only has the issue when idling, doesnt matter if its in park or in gear. anyone ever have this issue or can shed some light on where to look first???? fuel pump had been recently changed just prior of me buying the vehicle
 
First off, did you pull the codes? You should only be getting a code 12, which is to let you know the OBD or On Board Diagnostic system is functioning... Also, the computer has a relearn time period, and if I recall it can take over 20 minutes of driving after you reset the computer to adapt to the sensor readings.
Next, check all your plug wires for cracking or burns from the headers. Check all of your vacuum lines for leaks. The throttle body base gasket is a common area for vacuum leaks, might consider getting a gasket set and rebuilding the throttle body. It's about $50, get the one with the fuel pressure regulator diaphragm. It looks harder than it is, took me a couple of hours and a few cans of throttle body cleaner.
Assuming all of that is operating properly, I would check your EGR solenoid and EGR valve... Quick way to see if there is a problem with either one is to remove and cap the vacuum line going to the solenoid. It's not the proper way to test it, but will give you an idea if it's functioning at all.
Try the free stuff first, and post up what you find. Lots of knowledgable guys on here.
 
Yup, check all that, except that the EGR only comes into play over about 50 MPH, so that is probably not an issue.

Check your PCV valve.

Check your cap and rotor. Replace if they are noticeably worn.

I am assuming you installed the O2 sensor the correct distance from the head, correct? If it's too close or too far away it won't work right.

But pull the codes and see what is going on before you start tearing into things. The codes will save you a lot of time, effort and money.
 
Actually the EGR will cause a stumble at idle or in low rpm situations. Mine did, it was sticking ever so slightly and would cause a miss at low rpm, and almost stall the motor when I would stop at a light, or sometimes while going around a corner. I think if the solenoid goes bad it basically becomes a giant vacuum leak... We should make a sticky for this topic over in the Injection section or something, this comes up a lot.
 
Jessie James touched on it: Where did you install the sensor? You really should consider a 3 wire heated sensor as they provide a much more consistent output, especially with headers.
 
One thing I recently found on my new-to-me '90 TBI was that one of the four colored wires that tell the throttle bodies what to do was only attached by one strand.

I had a weird intermittent idle issue and seems to have cleared it up. I was surprised the issue wasn't worse. Never threw a code, and never felt weird under throttle.

Just throwin' that out there.
 
Keitha said:
Jessie James touched on it: Where did you install the sensor? You really should consider a 3 wire heated sensor as they provide a much more consistent output, especially with headers.
Click to expand...
I installed a 4 wire O2 sensor. A regular O2 sensor will not get to a high enough operating temperature with headers.
 
I have a set of long tube headers. the closest i could get to the head was right before the collector. so that where i installed it. the trucks great when driving. the 02 sensor made a big difference. i caped the vacuum hose to the egr but it made no difference. i drove it like that for a whole day. I'm off today so im gonna really dig deep ino this and see what i can find. i did notice that the idle is nice and smooth when the engine is cold, but once its reaches normal operating temp is when it has this stumble. and this is ONLY when idling. it doesnt give me any trouble what so ever when i initially accelerate. i appreciate all your guy's help.
 
Fist off, Don't replace parts to diagnose a problem.

I believe by your description that we are dealing with a rough idle.

If the engine runs smooth while driving and rough while idling you can pretty much rule out the ignition system as the load on it is MUCH greater while accelerating than idling.

Number one cause of a rough idle is a lean mixture.

Quick test for a lean idle mixture, spray a quick shot of carb cleaner into the throttle body while the engine is at idle. If it smoothes out for a second or two you are on the right path.

If you were having the issue with the O2 sensor missing or unplugged, it is not the cause of the issue.

Lean mixture can be caused by a number of things to include but not limited to.
Low fuel pressure
Almost all TBI vehicles have fuel pressure somewhat lower than what they were calibrated for. If it is low enough to cause a rough idle you will likely have a huge stumble on acceleration.

Leaky EGR valve.
This one may not be caught with the carb cleaner shot because it is not leaking clean air but rather exhaust gas. A leaky EGR valve can actually make your engine appear to run rich because some of the clean air is being displaced by exhaust gas.
See if you can operate the egr valve by pressing (squeezing) the diaphragm with your fingers. It should open up and then smoothly close. This movement does not rule out leaking but if it is seized up it is almost definitely leaking.

Manifold leaks around the throttle body cause a high idle.

Manifold leaks around individual runners can cause a rough idle but will likely smooth out with the shot of carb cleaner.
 
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wow, thanks for all the input texas. to give a better description of my idle, its not high and its not low. its actually very smooth for the most part but about every 3-4 seconds it has a hickup. it will just purr like a kitten and then have a stutter, purr again for like 3 seconds or so and then stutter, so on and so on. its not a constant rough idle. i will try to thoroughly mess with the egr valve again today, unfortunately we are having some bad weather right now in southern arizona. its VERY windy, cold, and starting to rain and i dont have any kind of cover, or garage to park in to work on it. if the weather doesnt permit, then i will try again tomorrow. thanks again for all of your help guys
 
That's a horse of a different color, does not sound like an EGR issue.

First off I'd check for a nice steady pulse of fuel comming from the injectors at idle. Looking for a change in fuel deliver that associates with the stumble.
 
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Similar symptoms to my throttle body wire damage issue. One strand 100% connected, the rest mashed together with some degree of continuity it appeared.

All nicely hidden under some manked-out electrical tape... :doah:
 
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