So I am getting to the point that I think I am about to drive the truck off a mountain and add 10 gal of gas to the equation. I have scoured the internet (including here) for any solution even those that seem improbable, but to no avail. This is the last try before I just scrap the vehicle.

Base Stats:
1991 Toyota 4Runner
3.0 v6
5 speed Manual
no performance upgrades (with regard to motor/electrics)
140,000

Symptoms:
Anywhere between say 20 and 300 miles while cruising (can be cruising at 65 in 5th or downtown in 3rd) the engine appears to die and miraculously restart all in less than one rev. The vehicle obviously jerks and the engine then picks back up right where it was before the annomoly. As best we can tell, the CEL does NOT come on. (Immediately after the "jerk" we pull codes to no avail.) The tachometer will drop for say 2500-3000 (where I usually keep it during cruising) to around 500 and immediately jump back up to 2500-3000. There is NO WAY, the true engine RPM is dropping to 500 as there is no discernable driveline slippage (NEW CLUTCH.) So in 5th gear at 500 rpms, the vehicle would not still be at 60+ mph and it is as I haven't died yet from the jerk. It is just as if the ignition dies for a brief moment and then comes back to life. (Or the speed of the vehicle itself in gear is effectively push starting the engine back up.) The engine will run fine for another 20 to 300 miles with absolutely NO syptoms. It is VERY HARD to diagnose.

So here is what we have done.
Timing belt & flex lite electric fan replacement at purchase (don't know if previous owner had issues with jerk, but vehicle had 130,000 and fan shroud was gone)
Engine seized while off roading in Mexican Mountains (an adventure unto itself)
Towed vehicle to US, dad rebuilt engine noting every spec.
New engine starts no problem
Jerk continues (we noted it on trip to Mexico)

Replaced spark plugs
coil
ignitor
spark plug wires
(Still Jerked)

Did Big 3 ground upgrade
Upgraded master (+) connector
Upgraded and cleaned all battery cables
New battery
checked master fuse box and wired with thicker guage
(Still Jerked)

replaced flex lite (when burned out controller led to engine overheat) with new factory clutch fan shroud assembly
(Still Jerked)

replaced distributor
(Still Jerked)

replaced ECU
(Still Jerked)

traced schematics to link between tach, ignitor and A/C amplifier
removed a/c amplifier (no a/c but cool enough who cares)
(Still Jerked)

Bypassed Emmissions Controls

We have inspected all vacuum lines, ran numerous continuity tests. Looked at more schematics than we can think of. This is so annoying and really makes me doubt I can trust vehicle for another adventure.

Please ANY ideas???

JJ

 

Oh and just about every item we have replaced has spec'd out fine.

 

Look at your fuel delivery system. I had a clogged fuel filter on one of my previous vehicles, and it would try and rip the back end off when it starved the motor.

 

Oh went and drained tank (was fairly clean) and changed filter. Good try.

 

How do the injectors look? Fuel pump? Distributer?

 

I have a 90 Runner, and I experienced something like your saying. I found out that it is the wireing that goes throgh the fire wall on the drivers side, you know, the wire pack that goes to the front of the engine. I was driving with my girl and my rig started sputtering, I pulled over and lifted the hood, while the engine was still running and thought my plug wire had come loose again. Nope, I moved the wires I explained and the truck started to cut out, I then put the wires on the master cylinder, kind of drapped over the front of it. I have not had a problem since. I know I need to replace those wires, jope this helps.

 

I'm with the above poster - maybe some kind of large temporary electrical short...

 

-New distributor
-Injectors have been cleaned and spec out
-we have not found a mechanic with the correct banjo fitting to test the fuel pump when on the rig, but I would not expect such a sudden jerk only to get back to running less than a second later and with no subsequent jerks for some time (not at all like clogged fuel filter)

Now random electrical short is a possabity, but we never can trace it. All visible looms have been inspected and look good. We've run continuity tests on as many as possible. The jerk is a one time hit and then fine and I have never noted the problem when driving on uneven/off road at speed which I would think would cause the jiggle factor to increase.

 

Oh and thanks guys for input, not trying to sound frustrated. We have had this truck 2 years and I haven't really been able to rely on it yet.

 

Try doing the continuity tests with the wires in different positions, could be a broken wire inside but not visibly broken, like I said I have not got to the wires yet, but they still sit on the M.C. and have no problems, so far. Hope you can narrow down the problem.

 

I find that it always helps to be well hydrated while trying to trouble shoot.

 

You don't mention it but does anything else electrical go out when the engine sputters?

Does your sound system cut out briefly or the dash/ headlights cut out when it sputters?

 

Ah great question

ALL other electrics are fine. Radio/CD keeps going, lights don't dim at all. No CEL as if main bus lost and ECU in self diagnosis. Nothing other than tach dropping and immediately coming back up.

 

How about you give it to me, Ill take it off your hands so you will never have to worry about it...

 

I don't think this necessarily excludes a short.. You could be shorting something in the EFI relay circuit.. Bad gound on the injectors, perhaps sensor ground also, although I don't know the 3.0 that well.

Battery cables in good shape, etc?

Problems that occur infrequently can be a real pain.