Have been fighting this for months now and am out of ideas. Truck has been stumbling under load for over a year now. Started as an occaisional minor annoyance, now will barely get to 50 mph on flat ground. Starts up and idles smooth, but the slightest load and it bucks and sometimes even backfires through intake. No engine codes. Fuel system seems fine w/ good pressure at all rpm's, all injectors firing. Have recently replaced tps, maf sensor, distributor, cap, rotor, wires, coil, timing belt less than 1 year old, egr tested ok, checked for vacuum leaks, vacuum at steady 20 inches at idle, intake and throttle cleaned, compression at ~160 within 10 pounds for all cylinders. Tired of throwing parts at it and am running out of things to test. Any ideas?

 

Spark plug gaps too wide?

 

Does the problem go away if you bump the timing up to 12degrees?

 

Checked all the plugs and timing. Timing at 10 degrees and doesn't make much much difference when advancing or retarding.

 

Dang! Something is wrong.... but I guess you knew that.

Can you get it to stumble when you blip the accelerator from under the hood?

The '94 has no maf, so when you say you replaced it, I assume you meant the AFM, right? That is an expensive part. But it has no "sensor", only a pickup which detects the angle of the air gate.

The TPS is easy to get in incorrectly, as you know. I'm assuming you are sure its in correctly.

I'm still thinking the timing may not be correct under load. Don't know of a way to check that.

Just thinking out loud here.

 

have you checked the timing of the timing belt....it could have jumped a tooth. Might also want to have a look at your fuel filter and the gas tank's fuel pump's sock... something is clogged.

 

Can't get it to stumble from under the hood. The toyota dealer took a shot at diagnostics on it and said that the AFM was bad and they could replace it for _only_ $1000. I found a remanufactured afm for $180 and replaced it myself. Later I found some info on how to test it with a voltmeter and the old one was just slightly out of range but imho probably not enough to cause any issues.

It's certainly possible that I screwed up on the tps adjustment. I'll pull the throttle this time and check it again.

Also had the timing belt off and reinstalled checking both the marks on the gears and the paint spots from the original install. But may as well pull the cover and check that again anyway.

The fuel system has been pretty much vetted end to end. Pulled the gas tank and cleaned it out, replaced the pump, sock, fuel filter and pulsation damper. Pressure (steady at 40 psi and never drops below 35 when reving the engine) and delivery is good, tested from under the hood.

 

borrow a friend PCM and use in inplace of your origional.

Also check your grounds then.

 

I'll vote for the TPS.

And if you set it while in the truck, that's even a bigger vote.
that thing's a pain in the backside to set on a bench, about impossible to do it in the truck

It may even be bad...they do wear out.

 

Possible plugged Cat convertor. Try removing your O2 sensor from the exhaust and see if it drives better. Other things to check: Intake hose from AFM to Throttle could be cracked and seperating under load, bad/missing ground wire. Probably best to find a local shop that has a good driveability Technician and let him figure it out. (not talking about the dealer)

 

It was the tps. Pulled the throttle and found that one of the hex cap screws was about .001 too long and was bottoming out allowing the tps to move. Removing the throttle is faster than getting setup to adjust it on the truck anyway. Will never try that again. The effective timing was about 30 degrees past tdc -- surprised that it even started

Thanks all.

 

Is the TPS a throttle position sensor? and where is it? I just bought my 4runner and I'd like to know how to adjust it, because it seems that it is slugglish when I lay on the gas. It has delay, and acts retarded sometimes.
Thanks

 

Missing under load come usually from 2 areas, either leaking secondary ignition (all high voltage stuff) or plugged injectors giving you a lean missfire. It wont set a code in your case either. Have the injectors cleaned or flushed on the rig. If you flush them on the rig, only use a Motorvac? system. I havent seen anything else that works "on the rig" like the Motorvac does. (Motorvac is the same as the Snap On system cleaner)

 

, try what it did to my friends, pull all electrical plugs, even spark plug plugs, spray DW-40 in plugs, it sounds just like my friends the miss is not constant, could be just a electrical short

 

displacedAlaskan -- download it, bookmark it, reference it: http://personal.utulsa.edu/~nathan-buchanan/93fsm/

Makes working on your vehicle so much easier.