My truck has 75k mi on a full rebuilt engine (by dealer). Has always run decent, minimal abuse.

Recently it has had a lumpy idle. There is a light backfiring/popping at the tailpipe, possibly some knock. Loss of power, un-smooth acceleration...feels like someone feathering the brake erratically during acceleration. The idle varies between 600 and 1200 randomly.

Fuel filter replaced 1 1/2yrs ago, plugs/cap/rotor 9mos ago, o2 5mos ago. I checked for vacuum leaks with propane briefly, couldn't find any (not obvious ones anyway). EGR is blocked off and disabled (has been, and working fine for some time). Cat was replaced last summer. Air filter good.

It still has the original wires, but no signs of arcing....i don't think. The problems are all twice as bad when engine is cold. I also notice that when it acts up, the voltage guage moves back and forth ever so slightly. It stays in a high range, but just fluctuates(i estimate between 15-15 1/2 V). The battery is fairly new, and alternator works great, replaced brushes a year ago.

This is a mystery to me.

Today I started it cold (at night), and it died shortly after. It started after 8sec (pushing throttle) and idled harshly like "chug chug chug" until it warmed up, lots of deep low volume popping from exhaust.

H E L P ! !

i'll check for codes tomorrow. The CEL is on, but due to a recent tranny swap, and I left some tranny connectors disconnected. This didn't affect performance, and I'm pretty sure there are no new engine related codes.
I'll doublecheck.

 

Codes would help, but when you say original wires what do you mean? Since 75k miles ago? What kind of wires did you use?

Do you know how to check for arcing? Your problem sounds like it could be related to that.

 

no i mean ORIGINAL BABY from 1990 off the assembly line. Is that bad? Engine was rebuilt at 150k (pretty sure they re-used the wires), now its at about 225k on the odo. Wouldn't a problem related to that come on gradually, rather than all at once? I guess I have trouble understanding why a wire goes bad. I intend to change them though this weekend, I just don't expect that to be the cause.

 

wow yeah replace them wires that might clear it up a lil and no wires just go as far as i know or they start with a lil miss then all hell other then that check the air flow meter as well that might be a lil out of spec

 

Well, if they were REALLY good wires......maaaayyybe. But, carbon core from what I understand were OEM. Those don't last for pooooh. And, really, all it takes is for the cable core/s to burn through in a second and you're running badly.

They go bad because the core ....whether it's carbon or wire strand....and insulation eventually get brittle and break down from the heat and constant voltage just any conductor of electricity would. And some conductors are better than others. There's not much to an ignition cable. Gotta have low resistance and all.

How do the cable jackets look? Any dark discoloration/splotches? If they're arcing, it would be visible in the dark. Give that a try.

 

I've said that so many times......

 

or if you wanna have fun at night with a spray bottle on mist and just lighty spray the cap and wires and see if there arcing easy i guess lol

 

ohming your wires will tell you if they are bad. check the TPS as well. maybe temp sensor

 

Regarding the fluctuation of the amperage gauge: double check the connections (to include removing & scraping off any rust) of the "Big 3":

1. Battery to frame ground
2. Frame to engine ground
3. Alternator to battery hot

 

1 and 3 are good, i've never checked the frame to engine ground. Where is this located?

 

I checked the codes, 42 & 51. These are related to the tranny not being plugged up to the ecm, as I suspected they were the cause of the CEL....not related to my performance problems.

I put a meter on my plug wires..... left side was avg 15-18 kOhms, right side was 5-6 kOhms. Now I wish I didn't order replacements.
I suppose there is a good chance that after I replace fuelfilt, cap/rotor, plugs, wires tomorrow that it will still run like .

Any other ideas??

 

go ahead and replace the wires, i bet it fixes it.

 

The troublecode guide mentions nothing of the transmission being related to those two codes.

I believe the speed sensor is in the airflow meter and those two codes would encompass that. Test the airflow meter.

 

Actually, come to think of it, test the connections at the ECM for the TPS and VAM circuits.

 

No...you're right. The speed sensor is on the transmission. I was thinking of the airflow speed sensor. It seems to be all on the same circuit, though. Hmmmm....

Test the VAM.

 

Engine block, passenger side, towards front, black & yellow wire w/crimped on tab connector, bolted to the engine mount bracket.

Wouldn't hurt to check it I guess.

 

how do I test it?

as far as the speed sensor...its the vehicle speed sensor it refers to. My speedometer cable is detached, that is the cause of that code. (i put in a newer tranny with an electric sender, my old one had mechanical)

 

problem SOLVED!!!

when removing the air intake to get to the driver side plugs, I noticed a large hidden crack in the rubber right where the crankcase breather goes in. Major vacuum leak. Runs like a champ now

 

So....how did you fix it?

I fixed mine with several wrappings of electrical tape.

 

I basically did an ISR mod. Got a piece of 3" aluminum tube with a 60deg bend and stuck it in there nicely. I'll get some pics up tomorrow.