Vehicle surging, idling poorly

[Kurai]

*cough* Honestly I'm not trying to spam the forums with my questions; I'm just trying to track down the source of my problems before I start driving this truck.

1992 S-10 Blazer (Tahoe LT), 4x4 (doesn't work), 4.3L W series engine.

Vehicle idles roughly at times, smoothly at others. Today it performed a new trick while waiting in a drivethrough: Surging high while at idle. (Wheras before the 'surge' was a surge downward to the point of almost stalling.)

Things I have replaced already:

Plugs
Wires
Distributor Cap
Rotor Button
PCV Valve
Air Filter
Fuel Filter

Things I have performed already:

Oil Change
Remove and clean IAC Valve
Inspect and clean EGR Valve (I removed the air intake plenum to accomplish this but it went back on just fine)
Removed and cleaned the hose that connects the PCV valve to the air intake plenum.
Traced all vacuum hoses to where they join and none are loose or damaged under the hood (one is apparently loose inside the cab).
Filled with premium grade gasoline
Added fuel injector cleaner

Vehicle gives no diagnostic codes. (ECHU does, but that's the Antilock Brake light, not the Check Engine Light)

Is it possible to have a Throttle Position Sensor begin to malfunction, cause these symptoms, and not give a code?

[Schrade]

Vacuum leak, Kurai. Not easy to find, unless you can spot a disconnected hose. But I'm 99% positive - vacuum leak!

[Kurai]

Well that's what I think too, but I've seriously traced every vaccum hose in that engine bay at least three times now, and everything is just fine! No breaks, no deterioration, no disconnects. Plus I thought that a vacuum leak would only cause poor idle or stall, not surging?

[rlith]

Have you pulled the upper plenum and checked for washing? Have you checked your fuel pressure at the shrader valve? Have you pulled and had your ignition module (inside the distributor cap) tested? Is there a heavy layer of dialectric grease under the module? (If not, get some in there)...

Any of those could also be causing your problem,

[Kurai]

I pulled the plenum off the other day to get to the EGR valve. I'm assuming "washing" means that a place inside would be clean from a fuel leak? It was all mangy in there, so, no, no washing.

I have not yet checked my fuel pressure, but it was added recently to my To Do list as I found that two of the wires going into the modules mounted on the firewall were exposed and I had to splice them.

I just had a tuneup a little over a week ago with all new plugs, wires, dist. cap, etc.... would they not check for the proper amount of grease?

[samton]

have a 95 does the samething, replaced injectors, tuneup, fuel filter, alternator, thermostate, cleaned radiator, new catalitic convertor, new computer, still surges when slowing down...

[rlith]

Originally Posted by Kurai

I pulled the plenum off the other day to get to the EGR valve. I'm assuming "washing" means that a place inside would be clean from a fuel leak? It was all mangy in there, so, no, no washing.

I have not yet checked my fuel pressure, but it was added recently to my To Do list as I found that two of the wires going into the modules mounted on the firewall were exposed and I had to splice them.

I just had a tuneup a little over a week ago with all new plugs, wires, dist. cap, etc.... would they not check for the proper amount of grease?

Umm, you don't grease the shrader valve... That said, why did you take off your upper plenum to get to your EGR?

[Stiletto]

I HAD A 92 it started getting awful mph and exhaust rotted i could smell gas , turned out to be the cpi fuel lines i assume you have the cpi vortech? mine would surge up and down too if you find that out ask if a mech would just redo the lines or else it is a $400 fix also service eng would on Stiletto

[Kurai]

Originally Posted by rlith

Umm, you don't grease the shrader valve... That said, why did you take off your upper plenum to get to your EGR?

Yes, I know you don't grease the shrader valve. I never said I had done so, or intended to. XD

With my particular model of engine, the EGR valve is a sealed module, and is mounted underneath the plenum, and it can't be removed without first removing the plenum. Trust me, I wouldn't have done it that way if I could have just weaseled it out of there. It was a royal pain in the neck.

I don't think it's my fuel lines under the plenum, as I spoke with the former owner today and he told me that they'd been replaced a couple years ago after they cracked.

I think I may have my catalytic converter checked, since I don't know how to check it myself--sometimes it doesn't respond to gas well, and y old car did the same crap when it's catalytic converter was bust.

I have now noticed that the vacuum hiss in my cab DOES IN FACT lessen when I accelerate. So I KNOW I have a leak someplace that is bad enough that I can hear it in the cab, but not just by being under the hood. Dang, it's frustrating. I don't know how else to locate it short of removing my whole dash. -_-

[rlith]

I have the same motor as you do, actually easy to get the egr off without removing the plenuim, just remove the speedo/cruise bracket (2 8mm and 1 10mm bolt)

That said, I would try to find that vac leak before anything. You could even go so far as to bypassing the vent controls going into the cab and just keep a straight vac system on the engine.

Also, when you put the IAC back in, did you adjust it properly? If not that can also cause it because the end of it will dislodge and not move in and out correctly.

And yes, have your cat checked.

[Kurai]

Update here...

So, I took the truck to the mechanic to have him look at it. He said the only thing he could possibly think of is that A) it's the Throttle Position Sensor acting up without giving a code (which I had thought of and have now bought a replacement for) or B) the fuel pump, as he saw that the pressure was somewhat low.

I'll get my vehicle back from him tomorrow, at which point I'll install the new TPS; hopefully it will be that simple, rather than requiring a whole new fuel pump.

[rlith]

Originally Posted by Kurai

Update here...

So, I took the truck to the mechanic to have him look at it. He said the only thing he could possibly think of is that A) it's the Throttle Position Sensor acting up without giving a code (which I had thought of and have now bought a replacement for) or B) the fuel pump, as he saw that the pressure was somewhat low.

I'll get my vehicle back from him tomorrow, at which point I'll install the new TPS; hopefully it will be that simple, rather than requiring a whole new fuel pump.

What did he say the pressure was?