Solved 1999 Mounty erratic idle/low power at hiway speed

[slowstang88]

Well I'm posting here because my Mounty is slightly modded (welded thrush chambered muffler, 2 into 1 with 3" side pipe, and swiss cheesed lower air box). I bought the truck with 60,000 miles running really rough, poor idle, long cranking times, popping/sputtering/backfire through intake. I pulled codes and there were a lot and also tested fuel pressure which was extremely low (10psi KOEO). Replaced the fuel pump and cleared codes and CEL popped back on, was running 100% better but still not right. I got codes for IAT sensor out of range, oil pressure switch was also inop, and the ECT was also bad. Replaced the IAT/MAF, both of the other sensors and ran perfect! Well the modding bug bit so I hole sawed holes in the lower air filter box, ran even better and gave me a nice howl at 1/2-WOT but wanted more.. I was gonna get a Flowmaster but my buddy had a Thrush welded muffler on his truck and sounded good (and 1/2 the cost so i got one and cut off the catback exhaust and welded it on and made a 3" single out turned out the passenger side. About 3 weeks later it started acting like a dirty MAF so I took it out and cleaned it. Ran good for a couple weeks then one day started popping and missing off idle but past about 5mph ran fine, CEL came on again (lean bank 1, lean bank 2), re-checked fuel pressure which was fine, cleaned MAF again and cleared codes and took on a 20 mile drive. Standing acceleration was fine, but above 3000rpm started bucking lightly and had a hard time accelerating. I hooked up a Snap-on Modis and did a few functional tests (TPS voltage sweep, cylinder contribution test, watched o2 sensors, etc and everything was fine. Ran good for another week then CEL popped back up but with no driveability problems. pulled codes and got lean bank1 and 2 again and checked for vacuum leaks with carb cleaner, found one on the back of the engine. i replaced every brittle vacuum line with rubber (and tied them out of the way) and got ahold of the PCV valve which was only half in the gromett. Replaced the gromett and PCV and all was good for another week (till this morning). Cold start, hunting idle, 1100-500 for about 5 minutes until it warmed up. Hit the hiway and power was WAY down at 70mph, would downshift 2 gears to keep up so i pulled over and unplugged the MAF to fatten it up in case it was lean and it ran fine. No CEL, I'm getting tired of this damn thing...


So far I've replaced:

Fuel pump assembly
MAF
ECT
Plugs/wires/tested coil packs
Oil pressure switch
Muffler
Oil/oil filter/air filter

I did talk to the previous owner and he said his "mechanic" friend tried adjusting the timing and pointed at the cam position sensor, replaced the fuel pump (and didn't use fuel hose, which rotted out and caused fuel pressure loss). I'm about to park this thing, it's getting old having to think about it blowing up on my 50 mile drive to and from work...

 

[drdoom]

Put a fuel filter on there.

 

[slowstang88]

I always skip the simple ****, I actually did that and it cleared up for the most part. Still surging a little at WOT, but no CEL and no tip stall or backfiring through the intake... Thanks for the dumbass reminder.... lmao

 

[slowstang88]

Well I spoke too soon.. CEL back on for lean bank 1 and 2 again, still runs ok though.. I'm about to replace all of the vacuum lines... I'm thinking it may have a small leak.

 

[drdoom]

Could you put a smoke test on there? I agree you must have a leak or two.

 

[Turdle]

Check the pcv valve and grommet. It is way back on the lower intake manifold-

 

[slowstang88]

FIXED!! Well it was low fuel pressure, I checked it last because I replaced the pump/fpr/level assembly at the end of last year. KOEO fuel pressure was 22psi, 35 at idle and 25 WOT to 3000rpm. The rubber line from the fpr to metal fuel line had a crimp clamp that wasn't tight enough and was pissing out pressure in the tank. I replaced the 2 short rubber hoses with methanol safe hi pressure f.I. hose and doubled up worm gear clamps and it's back to its old self. 65psi KOEO, almost 70 at idle and 65 WOT at 4000rpm.

 

[drdoom]

Good job and thanks for updating.

 

[slowstang88]

Not a problem, if anyone gets lean CEL codes, check fuel pressure immediately. Running lean for too long can cause catastrophic engine damage or failure.

 

[Verse]

Not a problem, if anyone gets lean CEL codes, check fuel pressure immediately. Running lean for too long can cause catastrophic engine damage or failure.

Im sorry if I'm just retarded, seriously.. but what does running lean mean?

 

[Dave 1955]

Im sorry if I'm just retarded, seriously.. but what does running lean mean?

Means not enough fuel going into the motor. can be diff reasons..fuel pump weak..leaks..dirty filter.

 

[Verse]

Means not enough fuel going into the motor. can be diff reasons..fuel pump weak..leaks..dirty filter.

Ok thank you

 

[drdoom]

Yes there is an optimal air to fuel ratio roughly 15:1...lean means either not enough fuel or too much air I.e. vacuum leak. Rich is basically the opposite.

 

[slowstang88]

Running lean can cause detonation and extreme cylinder pressure/heat that can destroy pistons, valves, heads, and block decks.