2F Questions - Stumbling/Hesitating at 2000 rpm (1 Viewer)

[Hudson High]

I wanted to see if anybody has had a similar issue with their 2F:

My 2F recently had a valve job done, I've set the timing to 7 degrees BTDC, idle is at 650 rpm, mixture is correct, etc. It stumbles at 2000 RPM, no matter what gear I'm in. This only happens when my choke is off and the engine is warmed up. I can run 65+ on the highway with the choke pulled out, but once I push in the choke, it's like my engine loses 50% of its power.

-Possible root cause right now that I will check tonight is a broken BVSV, the lower one (pink, part # 90925-05046). I know that this operates the choke opener system, so I'm not sure that this is the source of the problem. This is being replaced tonight once the part comes in. If closing this vacuum leak fixes the issue, I'll let y'all know.

Any other suggestions? I read that the Hot Air Intake flap being closed could be the source of the problem, but I haven't verified that it's operating correctly yet.

Thanks,

-Hudson
1987 FJ60

 

[unklwedy]

90% of carby problems are ignition related.

 

[hodol88]

Classic: EGR valve.

https://forum.ih8mud.com/threads/fj60-classic-stumbling-issue-egr-valve.370757/#post-5375258

 

[FJ40Jim]

Classic coasting decel fuel cut problem. When the head went back on, one of the zillion vac hoses was misrouted, so now the decel fuel cut system always thinks the engine is coasting, so the computer turns off the slow jet above ~1800RPM.

 

[Hudson High]

Classic coasting decel fuel cut problem. When the head went back on, one of the zillion vac hoses was misrouted, so now the decel fuel cut system always thinks the engine is coasting, so the computer turns off the slow jet above ~1800RPM.

Jim,

That's what I thought too...until I corrected the misrouting. I found the decel hose was connected to the wrong tube, fixed it, and still have the same issue. That's what I get for letting someone else touch my engine...I got it back and it literally had 50% of the hoses connected wrong, along with a couple broken things (like my rear windshield wiper?!?).

Thanks for the advice.

-Hudson

 

[Output Shaft]

"Classic coasting decel fuel cut problem. When the head went back on, one of the zillion vac hoses was misrouted, so now the decel fuel cut system always thinks the engine is coasting, so the computer turns off the slow jet above ~1800RPM."

That is really good sleuthing there Jim.
What may also possibly be the problem (if it's not incorrect vacuum hose routing) is the vacuum switch itself. It could be stuck in the ON position. See attached for info how to test it.

 

[FJ40Jim]

The absolute way to rule out decel fuel cut (which is what OP symptom sounds like) is to make up a couple jumper wires to connect from battery 12V+ & ground to the idle solenoid on carb. Connect power & grd. to carb, verify that ICS clicks audibly, start engine, go for test drive.
If symptom is gone, troubleshoot decel system.
If symptom persists, then it's not a bad decel fuel system, ICS, computer, etc.

 

[Hudson High]

The absolute way to rule out decel fuel cut (which is what OP symptom sounds like) is to make up a couple jumper wires to connect from battery 12V+ & ground to the idle solenoid on carb. Connect power & grd. to carb, verify that ICS clicks audibly, start engine, go for test drive.
If symptom is gone, troubleshoot decel system.
If symptom persists, then it's not a bad decel fuel system, ICS, computer, etc.

Thanks fellas,

After doing some more investigation last night, I think that my issue could be related to the decel fuel cut-off. I'm going to troubleshoot some more today after work, focusing at that system. I'm having all of the symptoms, regardless of the now correct vacuum routing. IF my vacuum is still below 12 in., then there's probable cause for my sensor staying on.

I verified that it seems to be a fuel-related issue, because I was able to get up to 75 mph last night, but only when I pushed and held the pedal to the floor. Before any of these issues arose (and before the head rebuild), I could get up to 75 mph by depressing the pedal about 3/4 of the way to the floor. Logic says that this stumbling and immediate loss of power ~1800 rpm is directly related to the secondary jet not supplying fuel, and thus related to the decel fuel cutoff switch.

If any of this logic is not correct, please correct ME! I'm learning all about this engine as I go. Thanks y'all.

-Hudson
1987 FJ60

 

[Output Shaft]

Engine (manifold) vacuum at idle at sea level should be around 20 inHg not 12. If you're reading 12 at idle, there is a big vacuum leak somewhere.
The secondary in the carb may not be opening fully as well. It is highly likely if the carb has never been rebuilt, but it is not related to the decel fuel cut.

Also don't confuse a slight hesitation in acceleration (or sometimes a jerk) when at higher RPMs when you are cruising fast, mid throttle and then floor it. That 'jerk' is the pause and blast when the secondary fuel circuit opens and all that yummy extra fuel suddenly gets dumped into the engine.

 

[Hudson High]

Engine (manifold) vacuum at idle at sea level should be around 20 inHg not 12. If you're reading 12 at idle, there is a big vacuum leak somewhere.
The secondary in the carb may not be opening fully as well. It is highly likely if the carb has never been rebuilt, but it is not related to the decel fuel cut.

Also don't confuse a slight hesitation in acceleration (or sometimes a jerk) when at higher RPMs when you are cruising fast, mid throttle and then floor it. That 'jerk' is the pause and blast when the secondary fuel circuit opens and all that yummy extra fuel suddenly gets dumped into the engine.

Yes, my vacuum is reading 11~12 inHG at idle. One thing I haven't checked since the head job is my valve clearance. I'm guessing that an incorrect clearance can also be the source of a vacuum leak. Thoughts?

Also, check point for manifold vacuum is off the manifold, at the rear of the engine, correct? I don't know the name of this vacuum port, but it is behind the carb.

 

[Output Shaft]

Yes, adjust the valves.
If you spray some carb cleaner around the manifold and suspicious areas (or starting fluid or a stream of propane) you can often find the source of vacuum leaks.
The AC idle up port is a good spot to connect a vacuum gauge.

 

[Hudson High]

Yes, adjust the valves.
If you spray some carb cleaner around the manifold and suspicious areas (or starting fluid or a stream of propane) you can often find the source of vacuum leaks.
The AC idle up port is a good spot to connect a vacuum gauge.

Excellent. Thank you! I'll follow up with my results this weekend.

 

[LAMBCRUSHER]

vac leaks and valve lashes, oh my. the VCVs can go bad inside and be very hard to troubleshoot. I had the dizzy vent vcv go bad on me, took me down to 9 inches....once I found it, vac jumped back up to 16ish(high elevation)

 

[FJ40Jim]

The VCV's (there are 2) are definitely on the list of suspect parts nowadays. I can't remember the last time I saw a truck w/ both of them fully functional.

 

[Hudson High]

The VCV's (there are 2) are definitely on the list of suspect parts nowadays. I can't remember the last time I saw a truck w/ both of them fully functional.

Thanks fellas.

I did have an issue during the winter where my VCV would allow small backfires to come up the dizzy vent tube and into the cabin. Besides the terrible smell, it was annoying to hear the PFSST noise. I've heard that this symptom indicates that the VCV is faulty. That's the next part I intend on replacing.

After replacing my BVSV this weekend, I still read 12 inHg. I sprayed carb cleaner around all of the air manifold connections, egr components, carb, etc. and did not find any leaks. Valve clearances and VCV are next on my to-do list. I took the 'Cruiser to the ranch this weekend, and she idled well at 700 rpm, no rough idle issues, no overheating, so thankfully it seems that I'm heading in the right direction.

-Hudson

 

[Output Shaft]

A chronic low vacuum can also be the EGR valve that has stuck open to some degree.

https://forum.ih8mud.com/threads/oh-so-famous-idle-issues.854033/#post-9717404

 

[Slow Left]

I sprayed carb cleaner around all of the air manifold connections, egr components, carb, etc. and did not find any leaks.

Is the insulator/gasket (that goes between the bottom of the carb and the top of the intake) new?

A common source for significant vac leaks can be cracking in the insulator material around the pipe that comes out of it. Try the carb spray trick around that pipe and also around the surfaces contacting between the insulator/gasket and the top of the intake manifold and bottom of the carb. see image below (disregard the metal shield piece that broke off...nothing to see there...not relevant to the current conversation)

Also, another common place for vac leaks is off the PCV pipe. If you get a new, OEM PCV grommet (~$4), clean the OEM PCV valve in a plastic baggie with some SeaFoam overnight (then blow out with air) and reinstall with new hoses all the way back to the intake manifold and good hose clamps around each connection, you can rule that one out as a potential vac leak...

 

[Hudson High]

Is the insulator/gasket (that goes between the bottom of the carb and the top of the intake) new?

A common source for significant vac leaks can be cracking in the insulator material around the pipe that comes out of it. Try the carb spray trick around that pipe and also around the surfaces contacting between the insulator/gasket and the top of the intake manifold and bottom of the carb. see image below (disregard the metal shield piece that broke off...nothing to see there...not relevant to the current conversation)

Also, another common place for vac leaks is off the PCV pipe. If you get a new, OEM PCV grommet (~$4), clean the OEM PCV valve in a plastic baggie with some SeaFoam overnight (then blow out with air) and reinstall with new hoses all the way back to the intake manifold and good hose clamps around each connection, you can rule that one out as a potential vac leak...

View attachment 1068192

Yes, the insulator material is brand new, replaced when I had a head/valve job completed. I soaked that area and didn't notice anything. My old one was cracked prior to the job. I replaced the PCV valve and grommet about a month ago but didn't spray it to check for leakage.

 

[Slow Left]

I replaced the PCV valve and grommet about a month ago but didn't spray it to check for leakage.

Hmmm...should be good on the PCV circuit, then, if the hose connections are all snug...

12 in Hg vac on the intake manifold is low...should be ~19-21 in Hg at sea level. I would continue to interpret this reading as an indicator of a vac leak...keep thinking/double checking sources for large vac leaks.

Have you recently replace the intake/exhaust manifold gasket? If so, which gasket (manufacturer) did you use, where the manifolds planed before re-installation and have you re-torqued the manifold to head bolts?

Addressing the VCV issue is a good priority to have, since you have already identified the one on the dizzy as bad, and they usually aren't good if they are original...problem is you've either got to find good used ones, or a work around (appropriate component from a different make/model/year vehicle) that will do the same job as the original VCV.

Here is how you can test the VCV (see Emissions FSM):