89 XR7 wants to die/low vacuum after idling for 1+ minutes

Pablo94SC

Pablo94SC

Registered User
When I first start the car it idles fine. After sitting for a while (let's say traffic), it will start to stumble (almost like a miss), pull low vacuum (10-12"), and wants to die. If I hold the revs at 1200+rpm, rev it up above 4 grand for a couple seconds, or put the engine under load for a few minutes with a few minutes of driving, it usually clear and starts to idle fine until the next time I'm sitting at idle for an extended period of time. Sometimes just shutting the car down for a minute clears the issue.

I'm stumped. The only thing that I can think of is a vacuum line somewhere or perhaps the DIS. Everything else is new (cam sensor, fuel filter, plugs, wires, belts). Any ideas where to looks? Thanks
 
Last time that very same thing happenend to me it was a very small vacuum leak on the blower top. Do you have a raised top or recently re-installed it.
 
Thanks. I recently swapped back to an early model blower on the car, so that may be it. I'll pull the blower top off and upper IC tube to see if that's it. Hope its not the inlet plenum or EGR seal.

Just an FYI, the car ran flawlessly yesterday. Go figure.
 
The big nut on the supercharger probably came loose, or one of the supercharger intake tubes shifted and created a leak. Maybe one of the nuts backed off.

Open the hood and see if you can move the big nut with your hand.
 
Something is probably shifting/settling at idle causing the leak, then resealing after putting the engine under load (after it torques over). I wouldn't put it past the collar nut being the culprit. I left the SC top and upper IC tube together when I took them off last year, so I didn't think about resealing when I put the early model blower back on.
 
If you have EGR, it could be a bad EGR valve. Something like that plays more a role in engine operation during closed loop after engine warm up.
 
If it's EGR related you should also get a 'check engine light'
 
When the car does act up, it will throw a CEL once in a blue moon. Give it a quick rev to 3500rpm + and the CEL goes off.

Now, focusing on the EGR... wouldn't the EGR sensor (PFE transducer) throw a steady code?
 
Have you pulled the codes yet? And if the EGR failed mechanically, you wouldn't necessarily get any code set.
 
I haven't had time to do anything yet. By the time I get home from work (currently commuting to Stamford, CT from NYC), it's almost dark. Good news is she's been running well for the past two days. Bad news is I know it will happen again when I least expect it.

I remembered that I replaced one of my vacuum lines with a length of silicon hose (kind of thin walled) as a temporary fix when AZ was out of stock. I wonder if that hose or another is collapsing under excess vacuum* and staying stuck closed until I accelerate again. How exactly would I check that? I don't know if you can create excess vacuum conditions unless the engine is under load.

* I usually pull over 20" of vacuum when decelerating in gear.
 
dude those silicone hoses are thicker then teh rubber ones usually and dont collapse so easily. I say vacuum leak FTW
 
Car was idling fine, so I plugged the vacuum line to the EGR solenoid to take the EGR system out of the equation. After a minute, car started stumbling again. Again, after putting the engine under load (quick trip up the block), the engine went back to idling fine. This is with the EGR system completely out of the loop. I still may have a bad valve or solenoid, but its not causing the problem.

I had a stored ACT code, which I will change out soon, but it doesn't seem likey to be the cause. The car will easily make 14-15lbs on the stock gauge, and there is no oil residue around the connections, so I'm pretty positive its not the upper IC tube.

Any other ideas? I'm at a loss.
 
one other thing I noticed is when the car starts to stumble, you can smell the excess gas its dumping. Is it possible the fuel pressure regulator is crapping out? Would it throw a code? Finally, is it located on the back of the DS fuel rail, and how do I go about testing it?
 
Just pull the vac line leading to the fuel pressure regulator. If it has fuel in the vac line, the regulator is shot.

Start with disconnecting your battery for 20 minutes then reconnect to clear the adaptive memory tables. Start the car and let it try and find idle for 5 minutes. Then take it for a drive for 10 minutes.

See how it behaves after it relearns it's adaptive strategy.
 
I'll pull the vac line to the FPR and take a look.

Won't the car also reset the adapative memory after pulling the fuse to the EEC and cranking the car? That's the trick I've used on my '94.
 
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