Remote Diagnostic Time featuring Dad's '94 Toyota Pickup

So Dad's truck has a miss.

Its a '94 Toyota pickup, 22RE, manual, 4WD

Dad said:

The Toyota needs something, but I don't know what.  The engine continues to skip/miss, on an intermittent basis.  Sometimes it's almost never, sometimes it's nearly continuous.  I checked the plugs, no one is worse than than another, all had a good color, so it's not any one cylinder.  Suggestions have been: a worn shaft on the distributor (possible, it's original to the truck, 330K miles on it); the mass air sensor (Mike says that it rarely goes bad); the EGR vacuum modulator; and the igniter.  I don't want to just start replacing parts; the check engine light is not on, so there's no code to check.

I remembered that he had some issues with the wires at one point, and asked if he ever replaced them.  Also asked if there were any specific circumstances that resulted in the miss.

I started out replacing Toyota plug wires one at a time, thinking it might be one cylinder that was the problem.  With each, it would get better for a while, then start to miss again.  After all four were in, it didn't make a difference for long.  When I came back from Havan911 the other day, for the whole 7 miles it was missing and bucking.  But when I went to Publix yesterday, it only skipped a couple of times, both ways.  So, no, there is no circumstance that I can point to. 

Ok, since it was suggested, take the cap off the dizzy and see if there is play in the shaft?

Probably more non-helpful information, but today I when went to Publix, the tach needle was bouncing from the start.  It did seem to do it only when I was holding the gas steady, but not when I was accelerating, even slightly.  It got worse after I left to go home, started missing at all sorts of random times.

I checked the distributor shaft, couldn't feel any play from the rotor end, either lateral or end play.   

Some additional information that might be helpful:

This engine was put in at 128K miles; it was a rebuild from Knico engines in Moultrie.  Mileage is now 334K, so 206K miles on this engine.  Timing chain, tensioners, etc at 308K in Nov. 2019.  No, my notes don't indicate replacing injectors; they must be from the original engine.

So, who has thoughts on what to check?  Preferably in a troubleshooting manner, and not in a parts shotgun manner.

I wouldn't have thought dist shaft it is very short.

It sounded like a plug wire type thing. Sometimes you can see an arc at night.

Grounds can be an issue on older Toyotas - I've read suggestions which said clean /treat them all.

Tach needle bouncing could indicate ignition...

but what about fuel?  Pumps and relays go bad often

and, as L5 suggested, cleaning and tightening grounds

Anyone have any idea where the stock grounds are?  Just to help prevent him from searching everything?

jfryjfry said:

Tach needle bouncing could indicate ignition...

 

but what about fuel?  Pumps and relays go bad often

 

and, as L5 suggested, cleaning and tightening grounds

I think he had an issue with a fuel pump recently, but I might be conflating two different vehicles by two different family members.

Yeah the bouncing tach needle says ignition, at that age the tach likely runs on voltage spikes kicked back from the single ignition coil. I'd recommend replacing that coil.

Can he start the car at night and see if the plug wires or cap are getting  crossfire , 

and like a car a couple days ago , check the distributor cap and the center  electrode, and the rotor .

Good luck

Why not do it properly?

First, check compression and leakdown.

Then valve adjustment

Then ignition system

Lastly, fuel.

I watched someone shotgun parts at their car for months chasing an intermittent miss, fuel pump, filters, coils, wires, plugs, injectors, on and on. They were just about ready to put a timing chain kit in it (which it ultimately did need but only due to mileage - it was close to 200K) when they finally listened to me and checked compression - it had 60 lbs on cylinder #3. Burnt exhaust valve, it also had a badly worn camshaft on that same cylinder.

californiamilleghia said:

Can he start the car at night and see if the plug wires or cap are getting  crossfire , 

and like a car a couple days ago , check the distributor cap and the center  electrode, and the rotor .

Good luck

 

Says he just did and nothing.
I generally like to spray a little mist of whatever on them to see if they spark, not sure if he did that or not.

The wires are fairly new, from what he's said

How old is the 22's throttle position sensor?  Dead spots on the rheostat cause the computer to spaz out. The dead spots are usually idle and cruise, where the throttle spends most of it's time.

If it has been replaced, did he clock it? There is a resistance range that its happy at.

Has he replaced the timing chain guides? With that many miles, its broken. The flapping chain can really screw with the cam's phasing.

In reply to MiniDave :

Fair point.  I'll try and get up there with my compression tester (or he probably can get to his regular shop quicker)

The tach might be dead/dying. I've had this happen several times after some years on the column. It will short the coil input voltage intermittently and rob the spark. Only applies if tach actually connected to coil feed, probable on '94. Disconnect tach and try again.

If no improvement, perhaps splice a light bulb temporarily into the coil power and/or module power feed. Put the bulb somewhere he can see it while driving and see if it correlates with the misses.

Super easy way to check if it's a ground issue.  Jumper cables from the block to the negative terminal.  See if the problem changes.

Chances are it's the pickup in the dist or the power stage.

I'm leaning towards ignition because the "bucking" complaint makes my spidey senses tingle.

Piggybacking on my previous post, have him start the truck. When it starts missing, unplug the ECU. If it smooths up, its probably the TPS.

In reply to Appleseed :

Unplug the ECU?

In reply to Appleseed :

I assume you meant unplug the TPS? Unplugging the ECU would shut down the engine...

The TPS theory wouldn't explain the tach malfunction though.

By any chance is the problem worse when the weather is hot? I don't know Toyotas, but assuming the igniter is a similarly solid state type as Bosch ignition control modules from the same era, when they start to fail, in hot weather, the car will miss. The Bosch type will ultimately completely fail (ie, no start) until it cools down, then work again. You can test it by spraying cold compressed air on it. The tach signal on some variants comes from the module - perhaps this is how your dad's truck works, too.

 Or dying hall sensor?

Mr_Asa said:

In reply to Appleseed :

Unplug the ECU?

Crap. Not the ECU, the Air Fuel Meter.

The intermittent nature rules out a lot of things.

We need closure.  Was this fixed?  

Bouncy tach indicates possible ignition issue.

I'd still want to make sure the valves are not too tight, which can cause an annoying temperature dependent misfire.

So Dad was tinkering, trying to check as much as possible from the thread without replacing.  Found that the terminals that go from the box labeled "igniter" to the coil had some severe corrosion.   Cleaned what he could and went to the store.  One miss/bounce on the drive.

He's gonna clean them more thoroughly this weekend. 

Thanks for the ideas and help, guys.