I'm the proud owner of a 1993 GMC Z71. It's got the 5.7 V8, 4x4 and the 4L60e four speed overdrive automatic.

I need some help in trying to figure out a problem I'm having with it. It has a weird idle and it "hiccups", which is the best term to describe it.

Now it idles in Park and Neutral at 800 RPM, you can feel the hiccup sitting in either P or N but its not bad. When I put the truck in Drive or any of the foward gears the idle drops to 700 RPM which is consistent with my Chevy work truck. But the hiccup is worse and the idle jumps between 650 and 700 RPM; the needle moves erratic between the RPM. When the truck is in Reverse the idle drops to 600 RPM, again, consistent with my Chevy work truck. However, the idle jumps between 600 RPM and a stall. Meaning the needle flies all over the lower RPM range and will stall out.

The truck drives fine, all the foward gears are there and strong, reverse is there and is strong. The truck does not stall out in the forward gears. Which originally led me to believe it was a transmission problem. Things I've done to it thus far to attempt to correct the problem are, transmission service, tune-up (includes new cap, rotor, wires, plugs, fuel filter, coil, air filter, PVC valve and thermostat), oil change, fresh coolant, began changing out old vacuum hoses. The next thing on the docket is a TBI rebuild, I've got the kit just haven't had time to do it. The TBI has been sprayed down with cleaner and will be cleaned thoroughly when rebuilt.

Now, the weird idle is not always consistent. Sometimes its worse than other times. Sometimes the idle is bad in P and N when cold but it will clear up. It is always felt when in gear and the engine is under a load. I've sprayed everything down with ether looking for vacuum leaks and have not found anything.

Like I said earlier in the post, the truck drives fine, all the gears are where they are supposed to be and when the truck stalls it always restarts without a problem. I've only owned the truck for two weeks and have only driven it twice and both times it has stalled out in reverse.

I'm starting to run out of ideas and was hoping to get some outside input and hopefully get this thing fixed so I can start using it.

I'm a Chrysler guy personally. I've owned nothing but them outside of my company work trucks since 1999 and I'm not new to turning wrenches, built my Neon a turbo engine and have put together a few transmissions for it. So, I'm pretty good at getting dirty and figuring problems out but on this truck, I'm clueless. Any help would be greatly appreciated. Thank you in advance too.  

-FettsZ71  

Hey wheeler,i dont know much about cheebee's
but is there an idle air control valve? the Snoman or Mudb8 will set ya on the right path,or KeithPappa.or actually any one who hasnt owned a Dodge for the last few years.wishta could been more help to ya.Happy wheelin!  

Check for vacuum leaks. First thing to check is thick gasket between TBI unit and intake manifold. They are known to go bad with age. Many years ago I had strange idle problems that I finally traced to this gasket being bad.  

There is a idle air control but Rochester decided to put a brass plug over it so that one can not adjust it.

I've checked for vacuum leaks and have found none.

I do have the rebuild kit for the TBI and it does come with the base gasket. I've just been kind of frustrated with it because I've been working on it for a week to figure it out and I'm kind of frustrated with it now, decided to take today off from it.

I'm going to pull the TBI off the truck tomorrow after work see if that helps it.

Thanks for the quick replies and the help. I'll let you guys know how I make out with it.  

Make sure you do change that gasket!  

Thank you. It does appear to be the TBI. But, not for the gasket reason.

Originally I had thought it was a fuel issue. One of the injectors was clogged, I cleaned it and got it working a few days ago. Well, after pulling the TBI unit off tonight I disassembled the fuel injector and bowls and the regulator. Found a bunch of gunk in the regulator and around it. Also found that the filter screens on the injectors to be clogged. I rebuilt the injector side of it then began to attack the TB itself and found that the center hose on the front of the TB, the one that goes to the PVC is clogged solid. I used two cans of carb cleaner on it and still can't blow any air through it.

Tomorrow I will finish cleaning the TB then reassemble everything with new gaskets and put it back together and see how she runs.

Then I've gotta find the source of my oil leak.  

Well, I've still got the idle problem. Decided to delve deeper into the ignition system and discovered that the pickup is shot, the wires for it are rotted that as soon as I touched them the sheathing fell off.

I've changed the module, and I have a new pickup, question is. How do I disassemble the distributor to get to the pickup.

I've got the distributor out of the truck but I'm not sure how the darn thing comes apart so I can change the pickup.

Thanks.

-FettsZ71  

All you have to do to disassemble the distributor is as follows:

> tip distributor so you are looking at the gear end of the unit.

> rotate it until you see where the pin goes through the gear and the shaft of the distributor.

> take a small punch and a hammer and gently tap the pin out.

> at this point you will be able to take the gear off the shaft

> once the gear is removed the shaft will pull right out of the housing exposing the pickup coil.

good luck,
hope my instructions are helpful clear enough    

I'd spend a few bucks on a oxygen sensor while yer at it...they do not always give you a check engine light or a code when they are'nt working. especially in a '93, which is OBD 1. That can cause yer computer to believe everything is fine and yer really runnin rich or lean, which will eventually stuff up the cat.converter. replace that throttle body gasket snoman mentioned first.