Runaway acceleration on a carburated 1FZ-F

[t00manyuserz]

So as work slows down for a bit, I am catching up with all the things that need doing with the car... this time it's runaway acceleration. I thought for the longest time it was the throttle cable, but I've replaced it, and it does not seem to be the culprit. Note - the carburator, throttle cable and even the damn accelerator pedal are all brand new. This is a GCC spec 1FZ-F.

Symptoms (all of this is "sometimes", and all of it requires applying the accelerator at least once):
- when under load, the RPMs will increase by themselves, even after the accelerator is released. Sometimes increase and stop, and other times just spool up to the redline. Even on a totally cold engine.
- when this is happening, I feel the gas pedal go limp (so I know I'm about to get a surprise when I clutch in)
- usually, if I emergency kill the ignition, roll to a stop and restart the car, it will be OK (though accelerating may sometimes cause another RPM spike)
- fiddling with the accelerator pedal sometimes helps. I will, for example, shift into 5th so I don't actually accelerate, then hit the gas pedal a few times. That may actually cause it to reduce the RPMs so that at least I'm not redlining.
- when the car works normally, it works perfectly. 800rpm idle without AC, 1000 rpm idle with AC, accelerating and letting go of the accelerator drops back to idle.
- although I do not try this often, I am 99% sure that if I floor the accelerator - especially past the "afterburner" click at the bottom of the pedal - it WILL make the RPMs go crazy.

Today I had a bit of free time on the road so I did two tests: the first time the rpms spiked >2,000, I killed the engine and stopped the car on the side of the road. I checked the throttle cable - there was slack at the carburetor side. I checked the throttle bracket that the cable connects to - it was fully released. So I drove again, and the second time it started spiking, I stopped the car once again, but did NOT turn off the engine. There was once again slack on the cable - more than before - but the throttle did not seem to have returned to the zero position. I tried to make it return - it did not go back - but when I pushed the throttle a bit, it returned and the rpms dropped.

Essentially, I am tempted to say that if I keep acceleration to below about 20-30% of the pedal, I do not experience this issue too severely (but not always. I have had cases where I'll press the accelerator just a touch, and it'll catch and run away), but if I do a rapid acceleration, I am almost guaranteed to have to deal with it.

Would anyone have any advice on what I should debug? Any help welcome.

 

[jonheld]

Would anyone have any advice on what I should debug? Any help welcome.

It is not physically possible for a carbureted engine to "run away" without the throttle plate opening.
So your issue must be with either the throttle linkage, cable, or a piece of crap jamming the throttle plate.
The fact that you tried to move the throttle linkage and it didn't move should point you in the correct direction.

 

[60 toy ota]

Does your gcc have cruise control?

 

[BassTracker]

Is there a return spring on the carb linkage?

 

[t00manyuserz]

No cruise control. Yes, there’s a return spring - under normal conditions, pressing the throttle snaps it back in place when released.

I realise it can’t just accelerate by itself so I’m trying to think if there’s some vacuum that pulls it, or maybe secondary throttle that opens and gets stuck, or what.

I’m slightly biased towards “nothing mechanical” because the carb is new, so I’m tempted to blame something outside the carb itself (like vacuum piping) but I don’t know enough about it to say with confidence. Any venues of troubleshooting appreciated. I am thinking to run through the basic tests in the FSM (DP/FICB/TP) to see if this gives any ideas.

 

[jonheld]

I’m slightly biased towards “nothing mechanical” because the carb is new, so I’m tempted to blame something outside the carb itself (like vacuum piping) but I don’t know enough about it to say with confidence. Any venues of troubleshooting appreciated. I am thinking to run through the basic tests in the FSM (DP/FICB/TP) to see if this gives any ideas.

The throttle control on a carburetor is 100% mechanical.
Again, you tried to move the linkage and it didn't move. What's stopping it???
Have you disconnected the throttle linkage and tried to move the throttle by hand?

 

[BassTracker]

A vacuum leak will cause a rise in rpms. Might be a good idea to spray around the carb base and intake runners with starting fluid. If you get a change in rpms when you spray you’ve found your leak.

 

[t00manyuserz]

The throttle control on a carburetor is 100% mechanical.
Again, you tried to move the linkage and it didn't move. What's stopping it???
Have you disconnected the throttle linkage and tried to move the throttle by hand?

Sorry. What I meant to say was, knowing that the carb is brand new, I am biased to think that there is nothing _wrong_ with the mechanical linkages (on an old carb I'd be diagnosing bent, stuck, dirty, etc, but I am telling myself none of those seem relevant to this case since it's brand new).

A vacuum leak will cause a rise in rpms. Might be a good idea to spray around the carb base and intake runners with starting fluid. If you get a change in rpms when you spray you’ve found your leak.

I happen to be also working on a coolant leak which I thought wasn't relevant, so I didn't post it here, but there is some accumulation of coolant around the carb base as in the photo below. When they originally replaced the carb back in the Middle East (I had an unrelated issue with my original carb, and knowing that I was exporting the car, I didn't try to debug it and just bought a new one while I could), I am about 97% convinced they did not replace any of the gaskets that must be replaced at that time - since they never asked me to provide any, and I am not sure they ordered any (and I was too naive to ask). Could this be a linked issue, or should I keep them separate?

 

[BassTracker]

it could be related. Most likely that thing has some sort of coolant temp based fast idle. If that is malfunctioning it could cause weird rpm issues.

 

[BassTracker]

So potentially, vacuum leaks/gasket issues, fast idle issues, or both.

 

[BassTracker]

- when this is happening, I feel the gas pedal go limp (so I know I'm about to get a surprise when I clutch in)

- fiddling with the accelerator pedal sometimeshelps. I will, for example, shift into 5th so I don't actually accelerate, then hit the gas pedal a few times. That may actually cause it to reduce the RPMs so that at least I'm not redlining.

This is kind of telling though. Makes it sound mechanical. Cable/linkage related etc. Could also be the fast idle system depending on the type (i have no idea what it has). Some are pot style that will act on the throttle shaft itself. Others use a controlled orifice to regulate speed (like a idle air control motor).

Another issue could be the throttle bladed themselves i guess. Something loose, slipping etc.

 

[jonheld]

Sorry. What I meant to say was, knowing that the carb is brand new, I am biased to think that there is nothing _wrong_ with the mechanical linkages (on an old carb I'd be diagnosing bent, stuck, dirty, etc, but I am telling myself none of those seem relevant to this case since it's brand new).

So there's no such thing as a new part that is faulty????
C'mon now.

 

[t00manyuserz]

So there's no such thing as a new part that is faulty????
C'mon now.

No, of course it is possible. But I prefer to be optimistic and assume that in a 30 year old engine, assuming one component that is brand new and a plethora of surrounding components that aren't, it is statistically more likely that something else is causing the issue than the brand new component. It would really be crap luck if the carb was bad from the factory, especially since I've had it running for a couple of years now and there's no warranty on it or anything (it hasn't done a lot of kms since the car gets little usage, so my definition of "new" is by distance travelled, rather than age).

Some more observations today:
- when idling at "normal", i.e. 800rpm, the engine is stable as a rock. No hunting and no rpm changes.
- when in "bad" mode, i.e. when the idle speed is too high, it will sometimes hunt and sometimes change rpms drastically - could drop by 600, could go up by 200. Even where it idles at that moment, tends to hunt +/- 50-100 rpms, but randomly - not consistent.
- I tested a case where it was sitting at 2000 rpms and I was stopped. I did not touch anything other than the brake (car was in neutral) - turned off engine - waited 30 secs - restarted the engine, idle was back to 800. I specifically did not touch the accelerator or otherwise do anything to the car. So it's like it rebooted itself when the engine turned off. I don't know if this is a red herring or not, though.
- I stopped the car when it was idling very high, at 2000 rpms, and took this video below. You can see the cable has slack. Video isn't great, but I reached for the throttle, and tried to push down on it (as if to release it). It did not budge. Then I caught the corner of it with my fingernail, and I swear I didn't move it more than 1mm, and it immediately reduced the rpms to 800.
- I sprayed brake cleaner at the base of the carb, but no rpm change at all.

I mean, I'm going to order the insulator for the base and some hoses that sit around the carb, then take it off and see if I can visually identify anything - and I have my old carb to compare (though it's 30 years old, so it has its own issues...), but I am a little stuck at the moment. The fact that turning the engine off and on makes me wonder if it's truly mechanical (which shouldn't move just because the engine went off, should it?) and not a vacuum leak somewhere, but I am not sure where to hunt for that.

 

[BassTracker]

Your description of events give me a couple of thoughts. Before we travel through that wormhole lets try something.

Start the truck up and let it get up to operating temp. Whenever you are able to reproduce the issue, get in the drivers seat and quickly stab the gas pedal and release. Report back what it does.

 

[t00manyuserz]

Your description of events give me a couple of thoughts. Before we travel through that wormhole lets try something.

Start the truck up and let it get up to operating temp. Whenever you are able to reproduce the issue, get in the drivers seat and quickly stab the gas pedal and release. Report back what it does.

Just tried - of course, NOW it's refusing to do it (spent the day driving fighting with the accelerator most of the time, parked it in the garage an hour ago, and it's now behaving properly). But I've done this before and it will usually go as follows - press, feel the cable pull, revs go up, I release the gas pedal but revs won't drop. Then, one of two things happen:

- either it will stay at the revs where I released the pedal - and fiddling with the gas pedal will edge the revs higher, then higher, then higher, as if moving some sort of a limiter upwards;
- or immediately rev even higher up to the redline. I'll turn off the engine (panic mode) and it'll return to normal when re-started.

(I recognize it really, really sounds like the butterfly valve is sticking in the open position for whatever reason. But why would it return back when engine turned off?...)

 

[60 toy ota]

When it's surging/hunting, could you open the hood and look for what is and isn't moving in your linkage?

 

[BassTracker]

This carb has an electric choke and three vacuum pull offs as well as several vacuum connections. At least one of the pull offs look adjustable. I’m willing to bet that the carb, being new when installed, nothing was adjusted. Every one of those items are connected to the throttle linkage in one way
or another and if they aren’t adjusted properly this thing may do all kinds of weird stuff.

I have lots of experience with carbs. Holley 4150’s,Carter/Edelbrock, Quadrajets, Rochesters, etc etc. Unfortunately every one of them are different, and I have zero experience with this one. Without being there it’s near impossible to diagnose.

You need a manual for the carb. Go through all the installation and adjustment procedures. Take a look at the old carb (if you have it) and compare the settings. Start from step one and rule out all the possibilities.

 

[t00manyuserz]

When it's surging/hunting, could you open the hood and look for what is and isn't moving in your linkage?

That’s a good idea. The problem is it’s usually an inconvenient time since I’m somewhere on the road when that happens … but you are right, I should really look at it when it’s doing that. I’ll try to catch it in the act.

View attachment 4215670View attachment 4215671


This carb has an electric choke and three vacuum pull offs as well as several vacuum connections. At least one of the pull offs look adjustable. I’m willing to bet that the carb, being new when installed, nothing was adjusted. Every one of those items are connected to the throttle linkage in one way
or another and if they aren’t adjusted properly this thing may do all kinds of weird stuff.

I have lots of experience with carbs. Holley 4150’s,Carter/Edelbrock, Quadrajets, Rochesters, etc etc. Unfortunately every one of them are different, and I have zero experience with this one. Without being there it’s near impossible to diagnose.

You need a manual for the carb. Go through all the installation and adjustment procedures. Take a look at the old carb (if you have it) and compare the settings. Start from step one and rule out all the possibilities.

This is likely accurate. It was installed by a shop that dealt in older cars, and the Canadian who ran it (in Doha) was a well respected character and I have no reason to doubt their best efforts, but it is possible that indeed, their adjustments were minimal and it was left at defaults, whatever they were. I’ll dive into the 1FZ-F FSM once again…

 

[BassTracker]

Having said all that, it seems the choke isn’t adjusted properly and is staying closed and/or closing at random times.

Remove the air intake horn/hose/elbow or whatever is called so you can physically look at the choke and throttle blades. Whenever you can reproduce the symptoms again look at the choke blade. Is it open or closed?

 

[t00manyuserz]

Having said all that, it seems the choke isn’t adjusted properly and is staying closed and/or closing at random times.

Remove the air intake horn/hose/elbow or whatever is called so you can physically look at the choke and throttle blades. Whenever you can reproduce the symptoms again look at the choke blade. Is it open or closed?

Can I drive around without the intake connected? I ask because it’s harder to reproduce without load - while I did say it’ll sometimes run away to redline at idle, it’s much more reproducible when driving with load on the engine.