Assorted Engine Troubles: A Thesis and a Cry for Help (1 Viewer)

[Aloha80]

Hi everyone, and welcome to my TED talk.

1993 FZJ80 @ 339k mi.

TL;DR: I've had weird engine symptoms since I owned the truck in 2018 at 308k mi, generally involving random bouts of poor idling. Now the truck is having increasingly more frequent drops in RPM under throttle (in gear and neutral), causing lurching and jerking. I've fixed a lot of stuff and am at a loss.

As the magnanimous PO retained all service records back to the 1993 dealership sticker (!), I know this had been an intermittent issue that had plagued the truck for some time. The PO's response was to whip out his AmEx Platinum and hand it to a mechanic. The engine was fully rebuilt at ~270k (costing PO $18,772.44, if you were wondering). Based on my records detective work, I suspect the PO sold it to me at 308k mi because of the persistent but intermittent idling issues (described as "dying at stop lights" in the records), which may have caused it to repeatedly fail CA smog--a death sentence for an old beast like this.

Over the years, the truck has intermittently had weird idling issues, and has thrown a number of codes (generally 26 - running rich), which don't always stay on. But let's fast forward to today.

I recently completed a 12-hour road trip. The truck behaved mostly fine, but would exhibit symptoms of dropping out of transmission lockup at speed, causing shock load to the drivetrain and generally resulting in an unpleasant road trip experience. RPM would drop by ~300 and then jack up again, resulting in a big clunk. Not fun. This happened regardless of:
-O/D on/off
-"Lifting" the brake pedal with left foot (suspecting the brake sensor; brake lights never illuminated)
-Cruise control on/off
-Speed or gear (although the "danger area" seemed to be during the first "phase" of throttle and/or at 50-60mph)

Also, the truck failed to start on one occasion. Some tapping on the (new) EFI Main Relay and it started right up.

Otherwise, all systems normal.

Upon returning home, the truck has started exhibiting what appear to be similar/related symptoms, but with new conditions:
-I know when I first start the truck if today is going to be good or bad. The idle will dip ~300RPM or stumble within a minute of start
-Idle speed is sometimes excessively high
-RPM drop/loss of power seems to mostly happen at lower speeds
-Definitely has to do with throttle - besides the sometimes off idle, it exhibits the problem RPM drops only when accelerator is pressed (in gear and in neutral)

MIL/CEL now shows the following codes (if I counted correctly, which I think I did). I haven't seen these particular codes before, even with the aforementioned issues:
-12: RPM Signal
-71: EGR System Malfunction

As these are new codes, I did not test anything for them yet. On to what I have done:

Green Arrows - Things I have Checked or Replaced:
-Air filter is clean
-EGR valve control 25870-66011 new @ 333k
-Distributor rotary assembly 19102-61240 new @ 322k
-Distributor o-ring 19127-66020 new @ 322k
-Distributor cap 19101-66010 new @ 322k
-Fusible link 90982-08264 new @ 328k
-Main EFI relay 90987-02004-83 new @ 339k (although the one I replaced was new @ 330k, but it was aftermarket/Amazon)
-Transmission fluid level is good (not shown)
-Transmission fluid new and clean @ 323k (not shown; looks like it could be replaced fairly soon based on FSM severe service interval but I don't suspect that's the issue)
-Fuel filter new @ 312k (not shown)
-Vacuum lines generally checked and replaced, although not foolproof (not shown)

Transmission Lockup Factors Checked:
-Brake sensor - not a suspect as brake lights did not illuminate during problem
-Coolant temp sensor - checked function per FSM at ECU, voltage within spec
-Cruise control speed sensor - not a suspect since issue persists regardless of cruise control activation
-Throttle position sensor - checked function per FSM at ECU, voltage within spec

Red Arrows - Known Issues or Suspects:
-MIL/CEL throwing codes 12 and 71 (known)
-EGR valve itself (suspect)
-Throttle cable lubrication/setting (suspect)
-Distributor slight oil leak at one of the spark plug wire insertion points (known)
-RPM sensor (not shown, no idea where this is; suspect)

Prior to the new codes, my mechanical engineer and gearhead friend's #1 hypothesis was dirty/clogged fuel injectors, but I suspect the problem is upstream of the injectors.

In conclusion, my hope is that you, the good people of MUD, can help me figure out not only this latest issue, but maybe that you see a trendline here that I do not that might explain all of the above.

THANK YOU!

 

[thabruiser]

Ok I will bite...

Haven't done much diagnostic of older 1FZ engines, but when I saw the RPM signal code, the first thing I thought of was the crankshaft position sensor. (Can't remember if RPM is read off the distributor on these, or off the Crank Position Sensor) As far as EGR, that is a rabbit hole that you'll enjoy or dread making right. A Stuck open EGR will kill a running engine, make it die at a stop light, etc.

 

[voodu3]

Have you checked the timing?

If you don’t have visual emissions inspections I’d get rid of everything from the pair valve to the egr. You’ll be amazed how much better it will run.

 

[Malleus]

Since you said the idling problem persisted after the engine was rebuilt, I suspect the problem is in the ECU. I'd open it up and look for leaking capacitors. They can be replaced and you'll essentially have a new ECU, for far less than even a used one will cost you.

FWIW, the IAC cannot be validated using only an ohmmeter. You have to pull it and verify the mechanical operation. Checking the resistance on the stepper motor will indicate all is well, even if the plunger is stuck. I had this happen earlier this year to my son's '94. I doubt this is your problem, as I said above, but I offer it as a further data point.

To cross the EGR system off your list of potential culprits, you can temporarily replace the sensor with a resistor. That will keep the computer happy.

Permanently removing both the EGR and PAIR systems is popular, but the truck ran very well with them when it was new, so they shouldn't be pointed to as root causes of poor engine performance. At least not without verifying those systems at the component level first.

 

[Malleus]

Also, with respect to the transmission, the kickdown cable is the most likely suspect. You'll either have to find a used one, or have a cable shop make one, or at least a new end, for you. The OEM is NLA.

You mentioned you replaced the transmission fluid. Did you drain the pan only, or replace all 3 gallons? Not that this will likely affect your lockup; that's controlled almost exclusively by the cable. Lockup is speed dependent and the "speed" the transmission uses isn't the output of the speed sensor in the tailshaft housing, it's the throttle position. See the FSM in the Resources section if you haven't already.

 

[Aloha80]

@thabruiser I'll have to check the sensor that determines RPM. Not sure which exact sensor that is yet. I think EGR sounds like the culprit (or at least a culprit)

@voodu3 Haven't checked timing as I assumed it was fine. I'll have to do that. Although I have no compunctions about ripping the emissions stuff out (I've done so for old motorcycles), I'd prefer to keep it intact in case I move to CA at some point.

@Malleus Many good points. What would I expect to see for leaking capacitors at the ECU? When I tested the TPS etc., I didn't notice anything obviously wrong with the appearance of the ECU. What's an IAC? A stepper motor? To answer your question, the transmission fluid replacement was only a drain and fill. Good note on checking the kickdown cable--another component I wasn't explicitly aware of related to this issue/series of issues.

 

[Marvelicious]

Just throwing some poorly educated guesses in here, so my input is worth exactly what you paid for it, but I'm betting that RPM code is the root of the problem. If the ECU suddenly thinks the engine has stopped, it will not send appropriate commands to everything else, transmission and EGR included. The EGR might be malfunctioning, but I don't think it would cause a bad RPM signal. The problem could be inside the ECU itself, but whatever sensor provides that signal should be the first item you check - sorry, no clue which sensor that is. Let the FSM be your guide.

 

[RC51]

Try a new throttle cable, although yours looks brand new. I had similar issues, but my throttle cable was bare metal.

 

[Malleus]

@thabruiser I'll have to check the sensor that determines RPM. Not sure which exact sensor that is yet. I think EGR sounds like the culprit (or at least a culprit)

@voodu3 Haven't checked timing as I assumed it was fine. I'll have to do that. Although I have no compunctions about ripping the emissions stuff out (I've done so for old motorcycles), I'd prefer to keep it intact in case I move to CA at some point.

@Malleus Many good points. What would I expect to see for leaking capacitors at the ECU? When I tested the TPS etc., I didn't notice anything obviously wrong with the appearance of the ECU. What's an IAC? A stepper motor? To answer your question, the transmission fluid replacement was only a drain and fill. Good note on checking the kickdown cable--another component I wasn't explicitly aware of related to this issue/series of issues.

Leaking caps are immediately obvious. Any discoloration on the PCB, especially around the bottom of the cap, is a sure sign the cap is leaking. Also, check for bulging containers.

The IAC is the idle air control valve. It's immediately adjacent to the TPS (throttle position sensor). I misread your first post and thought you'd checked this. Unlike the TPS, which is a potentiometer, the IAC is a stepper motor, connected to a plug, which seals the air bypass in the throttle body. Typically at fault if the idle is low when warm, but OK when cold.

IMHO, draining and filling the pan is a waster of money. You should exchange the entire volume contained in the system. This includes the transmission case, torque converter, cooler lines, lower radiator tank and transmission oil cooler. Fortunately, the front pump will do this for you. You only have to pull the outlet line off the radiator and reroute it to a gallon, or 5-gallon, container. You then start the engine, and fill the case as the pump pumps out the old fluid. Takes all of 20 minutes. And, it only costs you the price of new oil. It does take two people, though, just to be safe. It can be done with one, but I wouldn't; running back and forth to start and stop the engine just wastes time.

 

[baldilocks]

I believe this thread includes pertinent info. Read the entire thing.

Tachometer Stopped Working

The tach on my '94 stopped working all of a sudden. I have searched and searched but can't find any help fixing the issue. All of the other gauges are working fine. Can anyone tell me how I might go about troubleshooting this issue? I have no idea if it's the signal into the tach or the tach...

forum.ih8mud.com

 

[GeoRoss]

Also, with respect to the transmission, the kickdown cable is the most likely suspect. You'll either have to find a used one, or have a cable shop make one, or at least a new end, for you. The OEM is NLA.

You mentioned you replaced the transmission fluid. Did you drain the pan only, or replace all 3 gallons? Not that this will likely affect your lockup; that's controlled almost exclusively by the cable. Lockup is speed dependent and the "speed" the transmission uses isn't the output of the speed sensor in the tailshaft housing, it's the throttle position. See the FSM in the Resources section if you haven't already.

What about 35520-60111?

 

[ZackR]

How does the wiring harness look where is passes the EGR pipe? If you haven't already I'd recommend opening up the loom a bit and checking for wiring damage or intermittent shorts in there. The heat from the EGR pipe causing issues with the wiring harness is a common issue (it happened to me as well and was difficult to diagnose).

That wiring harness connects to the distributor. Perhaps there's an issue with the wire to the distributor inside the harness causing CEL for code 12.

 

[Malleus]

What about 35520-60111?

More of Toyota's database purging nonsense. Last month, the later cables were the only ones available.

Better buy now, 'cause they'll be extinct in a week.

 

[Aloha80]

Thank you everyone, coming up with a long laundry list of things to check.

New as of today: truck won’t hold an idle. Super cool as I need to take my dog to the vet.

It’s colder today than it’s been in a while, that’s the only variable that’s changed.

Edit: allowed it to warm up with application of the throttle. It now holds a tenuous idle.
-Pulled EFI fuse (15A) to see if it would throw more/same/different codes.
-Upon reaching normal operating temp, it holds a shaky idle at about 600-800RPM. Looks like it’s slowly struggling to hit 850RPM.
-The above is with the A/C compressor on. Turning the A/C compressor off drops RPMs to about 600, where the truck gets shaky and the idle creeps back up to about 700RPM.

 

[GeoRoss]

More of Toyota's database purging nonsense. Last month, the later cables were the only ones available.

Better buy now, 'cause they'll be extinct in a week.

I‘ll count myself lucky, I just purchased this from Partsouq last week for the parts horde.

 

[LandLocked93]

Gambling to posit that your notion to move to CA is really the problem. lol

It is possible that the PO (or whomever, upon reinstall of the engine) screwed around with the base line adjustment of the accel cable at the TB.

Otherwise, dump the PAIR and disable EGR (loop-back method), replace the accel cable and perhaps the accel pedal assy too (especially if it's rather hard to depress).
If not done already, replace the PCV and hose thereof, inserting a catch can into that new hose (simply to catch future crud).

These steps resolved all the very similar issues I was having with my 93. No issues since, ~4yrs ago.

 

[TheBussman]

definitely clean/inspect the EGR valve, if its sticking open it wont idle right at all.

 

[Aloha80]

Well boys, found my newfound idle issue and certified myself as a complete idiot.

Was going about inspecting the throttle body and look what I found:

 

[Aloha80]

Well boys, found my newfound idle issue and certified myself as a complete idiot.

Was going about inspecting the throttle body and look what I found:

View attachment 3478583

Doesn’t explain the entire list of problems, and I still suspect EGR. I’m also 99% sure this happened while I was poking around checking things just before my original post. But at least the mysterious new idling issue is…no longer mysterious.

 

[manofthewoods]

Doesn’t explain the entire list of problems, and I still suspect EGR. I’m also 99% sure this happened while I was poking around checking things just before my original post. But at least the mysterious new idling issue is…no longer mysterious.

I'd bet it's been cracked for awhile. Sure, your poking around might have made it worse, but any leak is no bueno.