Intermittent rough idle when hot? (1 Viewer)

[cbmontgo]

My 1980 runs great...usually. Every now and then though, it has a rough idle when the engine is hot (like when exiting off a highway and coming to a stop sign).

Most of the time, it idles just fine (and always runs perfectly through the higher RPMs). This issue is somewhat rare, but I can't seem to isolate it.

It has new cap, rotor, plugs, plug wires, and fuel filter. All OEM. Vacuum hoses look good, and timing was adjusted not too long ago according to my service records from the P.O. All stock emissions equipment and engine.

Any ideas?

 

[edwjmcgrath]

Vapor lock? Did/Do you have a carb fan?

 

[cbmontgo]

Vapor lock? Did/Do you have a carb fan?

It does have a carb fan, which seems to help as compared to my '76 without one.

 

[subzali]

Carson, how is the fuel system? A quickish check is to pull the air horn off the carb and look for gunk in the fuel bowl that may be clogging the jets. That happened to me a few years ago, drove me nuts for a while.

Also, is your brake booster ok?

 

[cbmontgo]

Carson, how is the fuel system? A quickish check is to pull the air horn off the carb and look for gunk in the fuel bowl that may be clogging the jets. That happened to me a few years ago, drove me nuts for a while.

Also, is your brake booster ok?

I am going to check this out today.

As of this morning, the truck barely idles. This is starting to show all the signs of a dead idle solenoid at the carb (and no "click" when the key turns now). This would be an easy fix; I hope I am this lucky.

 

[1911]

As of this morning, the truck barely idles. This is starting to show all the signs of a dead idle solenoid at the carb (and no "click" when the key turns now). This would be an easy fix; I hope I am this lucky.

Since it started out intermittently, I would suggest that the Emission Control "computer" is the more likely culprit. The inputs to the circuit board are cold-soldered on and well known for cracking with age. The solution is to either re-flow the solder connections to the harness plug on the circuit board with a soldering iron, or just ground the white wire with a red stripe just before it enters the "computer", which will bypass the "computer" and turn the fuel cut (idle) solenoid on and off with ignition. I chose the latter.

 

[ColonelMustardFJ40]

I have a 76 FJ40 which is virtually stock and when the temperature gets into the upper 90's and higher, I have this problem as well. It shows up when I am idling for long periods like in traffic and it begins to idle rough. As soon as I get air moving through the engine compartment the symptoms go away. Additionally, when I shut it off, at these temperatures, and it sits for more then 20 minutes, it does not start as easily. It seems that whenever the carb gets heat soaked these symptoms appear. My understanding is that modern fuels cause these problems due to the low vapor pressure of the ethanol contained in gasoline. In Albuquerque this is 10% by volume. Use to only be in the fuel in the winter but now it is all year. My understanding is that while the fuel is under pressure the ethanol is not a problem (fuel injection) but as it enter the bowl in the carb, the pressure drops to atmospheric level and and with the high temperature, the ethanol vaporizes. Bottom line is that you may not have a mechanical problem.

 

[Fast Eddy]

My '76 2f has a similar symptom after coming out of high-speed driving; freeway. I feel like it's a vacuum leak that only occurs when the emissions computer is in a certain state. I'd love to have a solution.

Edit: My problem also includes what seems like a missing cylinder or two on acceleration when in this state. Possibly a cracked intake manifold under the carb??

 

[cbmontgo]

I have a 76 FJ40 which is virtually stock and when the temperature gets into the upper 90's and higher, I have this problem as well. It shows up when I am idling for long periods like in traffic and it begins to idle rough. As soon as I get air moving through the engine compartment the symptoms go away. Additionally, when I shut it off, at these temperatures, and it sits for more then 20 minutes, it does not start as easily. It seems that whenever the carb gets heat soaked these symptoms appear. My understanding is that modern fuels cause these problems due to the low vapor pressure of the ethanol contained in gasoline. In Albuquerque this is 10% by volume. Use to only be in the fuel in the winter but now it is all year. My understanding is that while the fuel is under pressure the ethanol is not a problem (fuel injection) but as it enter the bowl in the carb, the pressure drops to atmospheric level and and with the high temperature, the ethanol vaporizes. Bottom line is that you may not have a mechanical problem.

I also have a '76, and it does the exact same thing that you describe. The carb fan that came a year or two later seems to really help keep the fuel bowl from boiling when the engine is shut off on a hot day.

My 1980 seems to have a separate issue though, because the idle is bad (as of yesterday) even when the engine starts cold. Sounds exactly like my '76 did when the idle solenoid wire disconnected.

I ordered the part from Toyota today and should have it mid-late week and will report back.

 

[cbmontgo]

Well, my idle solenoid turned out to be fine. Now my vacuum is at 6 in hg. Can't seem to find a vacuum leak anywhere using the water mist method. Idles like it has a gaping vacuum leak somewhere. Any ideas?

 

[Pighead]

Now my vacuum is at 6 in hg.

Check your PCV valve...

 

[cbmontgo]

Check your PCV valve...

It looks okay (maybe even newish), but is there a good way to test it?

 

[kurtnkegger]

I was told that by shaking the PCV, if you heard a rattle, it was probably good. On the other hand, a PCV from Napa runs about 4 bucks...if you might think it is your culprit, changing it won't break the bank.

 

[miker]

as a shot in the dark...
awhile back, i had a friend's 1980 Heep wagoneer at my house to see if I could fix it for the exact same symptoms. intermittent rough idle expecially when slowing down or stopping from freeway speeds and sometimes it would die coming to a stop. turned out to be the EGR valve. It was sticking open. if this happens, it will cause a huge vacuum leak that you will never find by spraying carb cleaner or using propane, etc. if it is sticking, you can try tapping on the EGR diaphram "can" with a screwdriver handle or something similar and see if you hear any noise like the EGR valve snapping closed.

Anyway, the EGR can cause a large vacuum leak if its not closing. Something to consider.

 

[cbmontgo]

as a shot in the dark...
awhile back, i had a friend's 1980 Heep wagoneer at my house to see if I could fix it for the exact same symptoms. intermittent rough idle expecially when slowing down or stopping from freeway speeds and sometimes it would die coming to a stop. turned out to be the EGR valve. It was sticking open. if this happens, it will cause a huge vacuum leak that you will never find by spraying carb cleaner or using propane, etc. if it is sticking, you can try tapping on the EGR diaphram "can" with a screwdriver handle or something similar and see if you hear any noise like the EGR valve snapping closed.

Anyway, the EGR can cause a large vacuum leak if its not closing. Something to consider.

You may be onto something.

I decided to rebuild the carb today, as it probably needed it and I usually do when I buy a new/old 2F. Anyway, she idles great now, but the EGR is whistling pretty loud. Going to try to dig into this tomorrow.

 

[cbmontgo]

Problem solved. Vacuum is good again and running like new. Issue was carb related, and I am thinking that my idle solenoid was intermittent. Rebuilt carb and installed new solenoid yesterday. Whistling noise was carb insulator gasket. Installed a double gasket and reinstalled carb this morning.

Back in business, as they say.

 

[miker]

excellent! glad you found it.

 

[cbmontgo]

The problem is back. I drove the truck to work yesterday and the idle went to sh*t after a long highway drive and I had to choke the truck to keep it idling at stop signs.

By the time I got home, it was idling perfectly again.

Took out the EGR last night and decided to clean and oil it; reinstalled it.

Started the truck today and it idled and ran perfectly until I got to a stop sign (engine still cold) and now it won't idle at all. It runs MUCH worse since I cleaned and lubed the EGR.

Any thoughts? I think I am going to start by finding a good used EGR and eliminate that.

 

[Fast Eddy]

Any thoughts? I think I am going to start by finding a good used EGR and eliminate that.

You could just bypass the EGR temporarily. The later EGR will really make it run like crap. The earlier ones can't pass as much exhaust gas.

 

[cbmontgo]

You could just bypass the EGR temporarily. The later EGR will really make it run like crap. The earlier ones can't pass as much exhaust gas.

I was told that an FJ60 EGR would work, but is this incorrect?

Also, what is the best way to bypass it?