Rough stumble/rumble at idle - Need some ideas

[MoJ]

(94 1FZFE w/130k miles)
Last week as I was backing out of the garage the engine began to stumble and shake. It sounds/feels like it's running on 5 cyl. It does this in D or R with or with out the brake applied. The RPM's vary only slightly from idle with each quick shake. The shake increases in frequency with RPM. If I put it in N or P and let the RPM's climb a bit it starts to smooth out but still isn't right. It doesn't die. No CEL at first but after a few start cycles I got a code 25 - Lean. The truck was fine the prior weekend when I drove 250 miles.

Since this started I've replaced the plugs, wires, dist cap, rotor. Nothing out of the ordinary was found on any of the plugs and they looked fine for 30k miles. The main intake hose is new. I disected and carefully inspected the engine harness were it passes by the EGR. Everything looked great. (Not a fun job btw)

Other background:
Truck sits a lot. I usually only take it wheeling or to pick up stuff for the house on the weekends. The fuel might sit on E for a month. It's slightly above E right now. The head gasket was changed as PM at 125k miles. The TB was cleaned last year and most vacuum hoses replaced at that time. Truck has always been very reliable.

I'm heading out now to change the fuel filter but I'm doubtful that will fix it. Injectors are of course a possibility but surely there's something else I can do before tearing things down to that point.

Any ideas are appreciated.

Thanks,
Jason

 

[MoJ]

New fuel filter in. No improvement.

 

[MoJ]

bump, anyone?

 

[jfz80]

have you checked your battery connections? what about where the harness passes through the firewall?

sorry i got no solid answers but i can sympathize w/ your frustrations, im dealing w/ a similar issue and electrical gremlins are the worst. think im on page 10 w/ one response as its sort of a guessing game.

When you press the gas, does it stumble and drop RPM at that moment? and when you let off it recovers?

a ? for others: would there be more torque/twist to the engine while revving in gear vs N/P ?

 

[sseaman]

It may not be the solution for this, but in the future i would let it sit with a full tank rather than empty.

 

[chitown40]

I just had something similar happen to my truck after it sat for a while. After removing the TB and valve cover, checking all the valve clearances, and visually inspecting everything for any obvious worn parts, I buttoned everything back up, and fired her up. I added some seafoam to the oil as I suspected I had a valve sticking and wanted to help free it up chemically. At idle the truck ran like crap, but eventually started to smooth out but still had a mild valve tap. At higher RPMs it smoothed out, but still not perfect just like yours. After letting it idle for a bit and letting the oil pressure build up and hit all the top end parts, everything started to return to normal. No more valve noise at all engine speeds. This happened to me one other time a couple years ago after the truck sat for a while and smoothed out after it warmed up and oil got all the way up to the top end.

 

[MoJ]

Bump

Batt volatage is 12v off, 14.5 running.
MAF (VAF) resistance is per FSM.


As far as damaged wires behind the glove box/where the harness enters the firewall --- Where exactly is the damage occuring? Right at the ECU? At the firewall? The view with the glovebox removed is pretty poor, nothing obvious.

Any ideas?
Any way to inspect the injectors & circuit w/o removing the intake?

 

[hj60]

Yep. Pull the harness connector for each injector, one at a time, with the engine running. If one of em doesn't cause a drop in idle, that's the cylinder having the issue.

 

[MoJ]

Yep. Pull the harness connector for each injector, one at a time, with the engine running. If one of em doesn't cause a drop in idle, that's the cylinder having the issue.

Getting to the harness connectors with everything assembled isn't doable.

I did however remember that I picked up a mechanic's stethascope with a long probe so I followed the FSM procedure for listening to each injector. All 6 sounded the same (rapid clicking) and each varied with a change in RPM so I'm assuming they're all firing.

I also put in 10ga of fresh fuel and gave it a 10 mile run. No change.

 

[IBCRUSN]

I had the same issue for the longest time and it ended up one of the spark plug wires was bad, might have even been the coil wire. I'll assume you are using OEM parts. That was my issue. Within 5k of the purchase I threw a set of Beck Arnley wires at it and less than three years later they were crap. I went so deep into the manual that I was testing every component in the fuel and ignition systems multiple times convinced the not-so-old wires couldn't be the issue.

With regard to the injectors they might cause it but would have to be in bad shape to suddenly develop a miss. You can check the resistance of the injectors at the ECU. I don't recall if the FSM states a specific ohn value but if they are all consistent probably not a bad injector coil. Might I ask why they weren't sent out while you had the head off? Not as OCD as some around here?

Does it change with engine temp? Mine got worse as the engine got up to operating temp. Try checking the coil, dist coils and the injectors as you mentioned.

 

[MoJ]

Update for anyone searching in the future:

I gave in and took it to the dealer. #6 injector. Replaced and all is good.

 

[Riad]

Did the dealer (or you) check the injector wires that travel by the EGR pipe all the way to the #6 connector? I have heard of injectors going bad but 130K seems little early, unless of course you've got really bad gas running.

 

[450 DUDE]

Found this thread , Moj I was wondering the sam as Raid?
Did you remember if you checked the resistance on the injector?


The reason I ask is the symtoms Im having are identical as yours besides the CEL code #.

Thanks,
ken

 

[68fj40]

Might check the vacuum line from the egr to the intake manifold. Mine was completely plugged up. Cleared the line and the metal tube on the manifold and my ruff idle and stumble disappeared almost immediately. This is on a 93 which doesn't throw a code for egr not sure if the 94 throws a code or not.

 

[MoJ]

Should have update this many moons ago.....

replaced #6 injector and all is well