RPM drop in 1st under load (FJ60) (1 Viewer)

[vszUnclear]

I know there are a bunch of "hesitation/stumbling" threads and i've read everything I could on this topic but I'm still not finding the source of my problem.

87 FJ60 with about 148K.
From a dead stop as I accelerate in 1st gear there is a stumble or RPM drop between about 1000-1500 rpms depending on the situation. I can literally see my tach drop a few hundred rpms as I'm accelerating giving gas. Most noticeable under load (hills). Seems most profound when cold, but it does happen at all temps.

I don't get any hesitations in gears 2-4 or at higher rpms in 1st. It's just that one little hicup, but it's critical because I live in a hilly area.

This problem has existed since I bought the rig in 2012. I've fully desmogged (has cat, no HAC). I've done a number of things trying to find the sources of my issue: new vac lines, new manifold gasket, carb lean drop, new fuel pump, fuel filter, tried unplugging the tach, tried unplugging emissions box, compression is pretty good (140s), new spark plugs/cables, new recurved/rebuilt dissy, timing adjustments (on BB, 7 deg BTDC), intake vac pressure is 16.5, exhaust is about 17, sprayed carb cleaner looking for intake leaks.

I have not yet rebuilt the carb but it's on the list. Pretty sure my secondary diaphragm doesn't work but that doesn't seem like it would affect this.

Anyone with similar experiences or possible culprits to chase down?

Thank you all.

 

[vszUnclear]

Bump.
Still hesitates or hits a flat spot right around 1200 rpm from a dead stop.
This is really driving me nuts so any suggestions are welcomed.

Some updates:
- found an intake leak at my PCV. I guess the new OEM grommet and valve weren't seated all the way. Don't have new vacuum readings yet but it made a big difference in my performance (but the hesitation still happens).
- rebuilt the carb and secondary diaphragm last week.

I read that the fuel filter should be full of fuel most of the time. Mine is only 1/3 full, so maybe that's playing into it.

 

[gregnash]

Yeah fuel filter is something that could be showing an impending fuel pump failure. When looking at mine you can constantly see fuel in there at least 2/3 of the way up the filter if not more.

The other thing I was going to say was that if you are desmogged you should actually be running closer to 10*-14* for your timing. Check your timing and get it set a bit higher. You might find that it smooths out or the flat spot is gone.

So does this only happen under load or can you replicate by revving the engine to the spot where it hesitates?

 

[vszUnclear]

Thanks Greg.
The fuel pump was recently swapped. I wonder what else could cause my fuel filter to run low. Air trapped in there?

I've read a bunch of conflicting things on post-desmog timing so I wasn't sure. I'll advance it tonight to see if that helps. Easy enough and it may be as simple as that.

The flat spot happens only under load, hot or cold. If I rev at a stand-still, no hesitation. I can work through the hesitation on flat ground and keep it running. But when starting up an incline from a stop the hesitation doesn't recover and the engine dies. I have get back on flat ground to get going again. Which sucks because my driveway is in the middle of a hill. I can only go one way at the moment

 

[vszUnclear]

Update:
I advanced timing to about 14 deg BTC and it certainly smoothed out the idle and I feel like I have more power. Great suggestion Greg!

The added power helps but the hesitation/stubble remains...

 

[Mace]

14* is a lot of advance at idle. is that with the vac advance hooked up?

 

[Slow Left]

Sounds kinda like you've still got a vac leak somewhere. Do you have an updated vac reading on the intake manifold at idle? Did it improve after the PCV fix? What altitude are you at?

I'm assuming your dizzy rebuild and recurve was done by Jim C., right? If yes, then you can most likely take timing advance of the table as a potential cause (except the vac advance).
Are the vac advance diaphragms on the dizzy new?
Do you have a the BVSV and check valve in line between the carb and the primary advance (the inside one, closest to the dizzy) diaphragm?
Is that vac line attached to the correct port on the carb?

Is the carb OEM (Aisin)?

Big vac leaks are usually the manifold gasket, the base plate (aka insulator) gasket that goes between the intake and the carb, an open vac port somewhere, or a crack in the intake manifold itself.
I would go back and check all components that still use vac, and make sure (like you did on the PCV) that they aren't a source of vac leak.
Is the HAC still on?
Did you check to see if there was a crack in the intake manifold on the inside below the carb (have to have the carb off in order to check, then look down...the hole...?

I'm trying to think if there is a more general way to confirm that there is a vac leak, so you don't go crazy replacing things. I think I remember someone suggesting spraying (in short bursts) the combustible carb cleaner type into the throat of the primary barrel of the carb (with the air filter off) while the engine is running while giving it a little throttle, like say 1,500 rpms or around where you are seeing the flat spot. If you hear it rev, then there is a vac leak.
Does that sound right?

 

[vszUnclear]

14* is a lot of advance at idle. is that with the vac advance hooked up?

Mace - Yeah in my haste yesterday I forgot to unplug the vac advance. My exhaust went from stinking strongly of HC to practically odorless after advancing, so I'd assume I'm heading in the right direction.

Sounds kinda like you've still got a vac leak somewhere. Do you have an updated vac reading on the intake manifold at idle? Did it improve after the PCV fix? What altitude are you at?

Thanks Slow Left.
I'll do another round of carb cleaner spraying and seafoaming to try and spot any leaks. Intake vac reading is still about 17 after PCV fix.

I'm assuming your dizzy rebuild and recurve was done by Jim C., right? If yes, then you can most likely take timing advance of the table as a potential cause (except the vac advance).
Are the vac advance diaphragms on the dizzy new?
Do you have a the BVSV and check valve in line between the carb and the primary advance (the inside one, closest to the dizzy) diaphragm?

Is the carb OEM (Aisin)?

Jim C did the recurve and confirmed that my vac advance diaphragm is fine. I have the BVSV (violet) in place and correctly routed per the desmog guide. I confirmed it's working (closed cold, open warm).

Big vac leaks are usually the manifold gasket, the base plate (aka insulator) gasket that goes between the intake and the carb, an open vac port somewhere, or a crack in the intake manifold itself.
I would go back and check all components that still use vac, and make sure (like you did on the PCV) that they aren't a source of vac leak.
Is the HAC still on?
Did you check to see if there was a crack in the intake manifold on the inside below the carb (have to have the carb off in order to check, then look down...the hole...?

I double checked that area in the manifold under the carb when i rebuilt my carb last week. It looked fine. I'm going to spray with carb cleaner again though. I'll go through every remaining vacuum device and make sure it's working and not leaking and report back.

I'm trying to think if there is a more general way to confirm that there is a vac leak, so you don't go crazy replacing things. I think I remember someone suggesting spraying (in short bursts) the combustible carb cleaner type into the throat of the primary barrel of the carb (with the air filter off) while the engine is running while giving it a little throttle, like say 1,500 rpms or around where you are seeing the flat spot. If you hear it rev, then there is a vac leak.
Does that sound right?

It's a good idea - except that the hesitation/flat spot doesn't happen unless it's under load. Say I'm at a stop and I want to get going again, then it hesitates. But if I'm just revving the idle I don't get this flat spot.

 

[Slow Left]

Alright. So, it is an Aisin carb? There are several ports on that carb which, after having done the desmog, are still open to air, then that's going to be a significant leak. Double checking those kinda things is what I had in mind.

Other thing might be to go back and double check the torque those manifolds bolts (in the correct order so you don't crack the intake manifold).

Don't quote me on that general technique for checking for a vac leak, but it kinda makes sense to me. If, say, at a steady 2,000 rpms, you spray some of that combustible carb cleaner in there and it revs or improves, then that might indicate it isn't getting enough fuel, or too much air - ie vac leak. If the reasoning is correct, then while it won't tell you where you have a vac leak, at least it will tell you it's worth it to keep looking...
Hopefully someone else here at MUD can confirm or deny this logic.

 

[vszUnclear]

Slow Left - I see what you're saying now about the carb cleaner down the throttle. I'm too green to know if it's a conclusive way to spot a leak but I'll give it a shot - easy enough.

And yes, it's an Aisan carb. I have all the unused ports capped. It was doing this stubble thing long before the desmog though. I have replaced the manifold gasket, just recently retorqed it and they were all tight. Spraying carb cleaner around the gasket doesn't expose obvious leaks.

 

[vszUnclear]

I am learning so bear with me here... I installed a dash vacuum gauge over the weekend and I found that vac jumps/fluctuates around 16 inches at operating temp idle (worse fluc and lower pres when cold). Researching this, it appears to be a symptom of valve issues or misfire. (Yes, I checked my vac earlier but didn't realize the fluctuation meant something bad.)

I then did the simple tail pipe "paper" test, where I put a piece of paper against my tail pipe. It is pushing and pulling, not just pushing. That is, the exhaust is oscillating between suction and pushing air out. Apparently this also points to problems with valves.

Dry compression numbers:
140, 140, 138, 139, 137, 145 PSI
I have not done a leak down test.

I am hoping for some opinions on where to go next.
If it's valve fix time, do I need to remove the head to remove/replace valves?

A big thanks to everyone helping here.

 

[Slow Left]

Have you adjusted the valves? Here's a link that outlines the procedure:

https://forum.ih8mud.com/threads/fj60-valve-adjustment.489587/

 

[Slow Left]

You also might have seen this chart:

 

[Callahan Offroad]

slow left: That is a great little chart.

vsz, I would think that your exhaust sucking exhaust back in is not good.

Older engines require more frequent valve lash adjustment, follow the link above and check your clearances.

 

[Slow Left]

Yeah it's a neat little chart; helps at least give you an idea what you're up against... I poached it from a post I ran across a while back.

To address the other question that vszUnclear is trying to answer, technically you wouldn't have to remove the head in order to replace a valve spring [EDIT]. But, if you try adjusting the valves first, and then determine that there is a serious problem with one or more of the valves, you might just be better off removing the head and getting it rebuilt.

That said, you still have low mileage. I would expect with that mileage that a valve adjustment would take care of things.

If you run the valves (adjust them), see where that gets you as far as improvement, and then ask yourself what information do you need in order to justify removing/repairing the head.

You might consider doing a Seafoam treatment before running the valves, to remove any deposits on the surface of the valves.

 

[vszUnclear]

Thanks Slowhand,
Yes should have mentioned that I did adjust the valves a few months ago. Could be I screwed it up but I took a lot of time finding proper TDC and double checked clearances...

Thanks for posting that diagram - yours is a more complete troubleshooting guide then the one I was referring looking at.
According to your guide, #11 seems to describe my vac readings the best. It violently oscillates about 4" centered around 16". It doesn't appear to be a "regular evenly spaced down tick" like #10. It simply oscillates back and forth about 4" at a very rapid speed. When cold, the oscillation is more like 8-10".

When I coast/decel downhill in gear the behavior changes to steady at around 22". As soon as i hit the gas I get the oscillation again. Oscillation happens at idle too.

[Edit: Removed a few statements where I confused valve seals and guides.]

So perhaps my valve guides are worn.
Any other test I can do to confirm this theory? Dry vs. wet compression test?

 

[Slow Left]

The guy in this thread just went through a similar decision making process, but for a different reason...

https://forum.ih8mud.com/threads/here-i-go-removing-the-head.791840/#post-9019789

I'm gonna defer to the expertise of the guys in that thread, since they've got a better handle on this than I would.

I would think that doing a leak down test like you mention above would tell you that the valves are or are not seating correctly.

 

[Jerry D.]

They're talking valve "guide", not seal. The guide is a bushing pressed into the head in which the valve stem moves up/down. The new guide will have to be reamed out to properly fit the valve stem. Meaning a head off job.

Jerry D.

 

[vszUnclear]

Thank you guys.
Steeling myself (and reading all threads) for taking the head off job.
It sounds like SBC parts are the way to go, so I'll let the shop sort that out or refer them to the above mentioned thread. But I should probably order a head gasket.

 

[Slow Left]

Right, 13711A is a seal. The guide is pressed into the head (see image below - red arrow points to a valve guide, through which the valve stem goes, and upon which the valve spring and seal sit...)