Problem o' the day...

[KILLSHOTS]

I decided to smokescreen the neighborhood today and took the car out for a drive. I started noticing a slight stumble at low RPM, so subtle that I thought I might have been imagining it. But it got progressively worse. Then, I turned it off for 5 minutes and when I restarted it, it idled up to about 2K and hung there, idling somewhat roughly. A stab at the gas did not reduce the idle. On the way home it continued to stumble badly at low RPM, and felt fine when I nailed it. I was hoping the MAP hose came off but no dice. All vacuum lines are intact, no changes there, it does not seem to me like a vacuum issue. I have not filled up recently so it's not bad gas. The plugs, coils, injectors and fuel filters have all been changed.

Any ideas?

TIA!

[PhillipsLT5]

Dam you got bad luck, sounds like a vacuum leak with the idle issue

[mike100]

take a look at the fuel tank purge solenoid line (under the plenum). there is a hard line that curly-q's around in a circle and attaches on the plenum corner close to the throttle body. you'll definitely get the won't-idle-below-1300 rpm from that leak.

[KILLSHOTS]

Dam you got bad luck, sounds like a vacuum leak with the idle issue
Agreed, Phil...the high idle definitely indicates a vacuum leak. But nothing seems to have changed, as least with the secondary system. The pump is still acting exactly the same with the ignition ON as it was before this problem surfaced. I've examined thoroughly under the hood and can't find anything that has come loose. As far as I know, everything hidden under the plenum that involves vacuum is directly related to the secondary system which, as I noted, is not functioning differently. Besides, Bill just went through everything under there with a fine-toothed comb and he tells me that it functions perfectly.

This car has 16K miles and has been garaged its entire life. I hardly ever drive it, yet I seem to have a major issue surfacing roughly every 5 miles of drive time. The aluminum guts of an LT5 are obviously bulletproof but everything surrounding it is clearly ludicrously delicate. Sorry if I'm offending anybody out there but I need to vent. I'm not a mechanic; I want a driver, not a project.

[XfireZ51]

Send it to Chicago.

[Kevin]

Agreed, Phil...the high idle definitely indicates a vacuum leak. But nothing seems to have changed, as least with the secondary system. The pump is still acting exactly the same with the ignition ON as it was before this problem surfaced. I've examined thoroughly under the hood and can't find anything that has come loose. As far as I know, everything hidden under the plenum that involves vacuum is directly related to the secondary system which, as I noted, is not functioning differently. Besides, Bill just went through everything under there with a fine-toothed comb and he tells me that it functions perfectly.

This car has 16K miles and has been garaged its entire life. I hardly ever drive it, yet I seem to have a major issue surfacing roughly every 5 miles of drive time. The aluminum guts of an LT5 are obviously bulletproof but everything surrounding it is clearly ludicrously delicate. Sorry if I'm offending anybody out there but I need to vent. I'm not a mechanic; I want a driver, not a project.

right there with you buddy. new plugs, wires, coils, pumps, filter, vac spider, ac...and that's just what I remember off the top of my head and I've had 2 weeks of perfect running in the last 4 years. It's a great car but the vac system has to go. It's rubbish

That said:
I would be willing to wager that your vacuum leak is coming from the rectangular rubber block on the plenum that connects the EVAP and PCV system. The block breaks down and gets loose and causes a high idle.

Mike100, on the 90, the EVAP purge is attached differently than the 91-95 cars.

[1990 quasar blue]

Chris with KOEO how often does the pump cycle?

[PhillipsLT5]

right there with you buddy. new plugs, wires, coils, pumps, filter, vac spider, ac...and that's just what I remember off the top of my head and I've had 2 weeks of perfect running in the last 4 years. It's a great car but the vac system has to go. It's rubbish

That said:
I would be willing to wager that your vacuum leak is coming from the rectangular rubber block on the plenum that connects the EVAP and PCV system. The block breaks down and gets loose and causes a high idle.

Mike100, on the 90, the EVAP purge is attached differently than the 91-95 cars.

PCV hose original or ? if so its toast

[KILLSHOTS]

Kevin and Phil, Bill just replaced the rubber PCV block with a new piece from Jerry. Quasar, my pump goes quiet for about 3 seconds and runs for just a moment. I think I've finally made the decision to ditch the entire secondary system.

[PhillipsLT5]

Getting rid of system, I like it, Bill has a camera to look under plenum

[KILLSHOTS]

UPDATE: This afternoon, I went through and rechecked every vacuum connection that I could touch, including the entire PCV assembly and MAP. Everything fine there. I wanted to make sure that my plug wires weren't causing the stumble, so I removed them all and put a small dab of DE in each one and ensured they were tight. (Marc said it's HIGHLY unlikely that the plugs are actually fouling.) Also unhooked the battery and removed and reseated the PROM that I had just installed.

Fired it up, it raced to 2000 again and hung there for about 2 minutes. Finally dropped off and sounded like it was just about to stall, when it jumped to 1200, then dropped to 900, then 1200, then 900, over and over again. Went for a drive of probably 2 miles, and the light-throttle stumble was DEFINITELY still there. Hammered it hard a few times and brought it up to temp. Pulled back in the garage and it was idling perfectly smoothly at 700ish. Shut it down for a few minutes and restarted and everything was fine. Went for another quick ride and no stumble. I hope this gremlin is gone for good but they sure are frustrating when they're completely random, here one moment and gone the next. I'd feel better if it hadn't reappeared after I made those few little adjustments, only to then disappear for no apparent reason.

[Schrade]

It IS possible for wireless log on internet, for Marc (or anyone else) to look at operating parameters, real time. NO special equipment necessary, except internet on both ends.

[XfireZ51]

By any chance had you unplugged the IAC? Also, if not up to temp, it very likely will stumble. Be interesting to see how it starts again once its cold.

[Schrade]

... and I think the hesitation, that Marc says ALL LT5's have, is exacerbated by time, and having the wrong parameter being borderline - perhaps KS's case here...............................

[KILLSHOTS]

By any chance had you unplugged the IAC? Also, if not up to temp, it very likely will stumble. Be interesting to see how it starts again once its cold.
No, hadn't touched the IAC. Agreed, I'm anxious to see what it does when it becomes cold again. Although, when both the stumble and high idle first occurred on Saturday, it was 105+ outside and the car was FULLY warmed up.