2001 Montana 3.4 Start/Run issues
 

Our high mileage 2001 Montana 3.4 is acting up. Symptoms are:

-hard to start cold or hot
-surges up and down some when idling, occasionally comes near dying then overcompensates.
-no MIL codes
-my scanner shows extremely negative LTFT (-30) which suggests that it is running very rich
-O2's look good to me but I'm no expert
-Exhaust does smell rich but interestingly the vehicle is getting great gas mileage as it is
-Does not seem to be any power issues, misfiring, missing cylinder...
-Might be hearing a little sputtering/backfiring in the muffler at hot idle

My thoughts:
Too much fuel is probably not a fuel pump issue.
An open fuel injector? I dunno how to test for this. Hard to get to injector disconnects.
Fuel pressure regulator maybe? What is the output of the pump and what would I expect to measure at the Schrader valve on the rail?

More info:
Last year I replaced the lower intake gaskets myself. Went well. Ran great after. This year, before a long roadtrip, I dumped in a bottle of injector cleaner thinking I was doing a good thing to run it through on the highway. Vehicle immediately went limpish and had terrible highway driving and economy the rest of the trip there. Someone local had a nice Snapon scanner and did some "blow out" test and the vehicle began to run fine again. Thinking was that some crud got lodged around o2 sensor and the "hot/max blow out test" dislodged it.

And here we are. I'm starting to wonder if that "injector cleaner" killed something, prolly the fuel pressure regulator?

I'm wondering if your IAC(idle air control valve) is causing the issue. Sometimes they buildup with carbon and don't allow air to pass through very well causing cold/hard startups as you are experiencing.

Best way to fix it, is to remove the IAC and clean it and the hole where it goes into with carb and choke cleaner.

iac maybe, or vacuum leak. sounds to me like the engine is lean though. check the fuel pressure 50ish or higher

does it have any codes, what are the numbers?

Wouldn't hurt to check the fuel pressure regulator. Pull the vacuum hose off and smell for gasoline in the hose.

Was it a "safe for MAF sensor" cleaner?

WTH ... you mean I shouldn't be using Lemon Pledge?

Exactly. A non safe cleaner might clean a MAF wire to put things back to norm as it begins to degrade it?

CRC Guarranteed to Pass Emissions Test Formula. I had it handy for another vehicle and I ended up repairing that one before using it, so I used it for this one. Following directions, I poured in the bottle at half tank while on roadtrip. MAF never got any of the cleaner.

I'll check IAC.

I'll check the pressure reg.

I dont suspect vacuum leak, but such would make it run rich, not lean. I am going to water-spritz test the engine this morning looking for vacuum leaks. Maybe I popped off an o ring on a fuel injector.

I dont have a fuel pressure meter handy but I may run out to Harbor Freight and pick one up. Seems to me that low pressure wouldnt be running the system rich, so I dont think it's weak. But a failed regulator might be causing overpressure as well as leaking into vacuum.

I woudlnt expect a LEAN issue to cause the LTFT to go negative.

My gut says pressure reg and then dripping injector(s). Either of these could allow it to run great on the highway...

Yep. :thumbup2

vacuum leak lets in unmetered air which is lean.

you can rent a pressure gauge from az or advance for free