JLG 600A sputters and dies when hot - Gas Carburated

[DavidL]

Starts fine, runs good until it gets hot (normally hot). Then, like light switch, it starts to backfire, runs rough, and stalls. On restart it continues to misbehave. If I wait until it cools off, all is good again. Takes about 20 minutes of running before it acts up.
I bought this used, and found someone had bent the choke blade...I thought that was it! But cleaned the carb and straightened the choke situation and same. It does start better now...
The guy I bought it from did disclose that it had this problem, but didn't know how to fix.
Swapped the electric fuel pump to eliminate that.
Ensured the fuel filter was good.
Blew air back into the tank to ensure line was not clogged. Cleaned debris out of the tank. Fresh Fuel.
The Oil pressure sensors were miswired according to the schematic. I think that's now OK. From the choke blade situation, a hack has been working on this in history.
Propane tanks are shut off. I think it does same while on propane.
I noticed that inside of the tank, the fuel inlet seems to be "hinged" at the end in the tank...what could that be for? Interested to see if by chance this is clogging up the works. I will also next try to run it with the gas tank loose in case the tank vent is clogged.

I read posts about a revised ECU for the LRG-425...Hoping that isn't what I need (cost). But want to understand why Ford offered that up and to fix what symptoms. Does anyone have the part number of the kit or hopefully a link to the TSB / info on the upgrade?

In reading I see that the coolant temp, combination of the oil pressure sensors changes the timing curve within the ECU. That's why I am wondering if the revised ECU addresses this issue.

I replaced the flywheel / starter from a Ford Ranger. Exact match. Damaged I am sure from trying to restart when hot. Hoping the ECU also comes from a Ranger (cost)...any ideas?

Hard to isolate fuel from ignition as it usually happens when I am up in the basket

Thoughts?

 

[OFF]

Sounds like an ignition problem to me. Specially when you mention is does the same thing on propane and you've already covered all the bases on gasoline. If you want to be sure, you could "T" in a fuel pressure gauge to the fuel line before the carburetor. What about the fuel shut-off solenoid at the base of the carb's float bowl? Have you looked at it yet?

As far as ignition goes, How old is your 600A? Does it use a distributor (possible magnetic pickup issue) or a coil pack/ECU (crank position sensor issue) ?

 

[DavidL]

Thanx OFF,
I have put a fuel pressure sensor on it, but can't read the gauge 40 feet up in the air
I have taken off the fuel shut off solenoid and ensured it was clean and easy to move. Also cleaned it's port when I had the carb apart and blew air through it.
Electrically I think it's ok too.

This 600A has a multi-coil pack with crank position sensor. This one was just prior to when they switched to EFI.
I might need to enlist the aid of a buddy to go in the basket (and get my work done for me ) while I run some more discrete isolation diagnostics (like the fuel pressure).

Was really hoping someone had the info on why Ford upgraded the ECU and to what.

I think if I was on the ground when it was happening, a timing light and simple dwell meter would be helpful to see if the timing is going nutty or if the ECU is misfiring the coil pack. Certainly an oscilloscope would be better, but don't have that in the tool box anymore.

Oh...I also rewrapped the exhaust with heat kevlar as the original was missing...the hope was that this was making the electronics warmer than they like...no change...it was a hopeful thinking repair...

Thanx for the feedback.

 

[OFF]

DavidL, "most" Ranger replacement parts are usable on those engines. The problem you are having is very often the crank position sensor. They are notorious for crapping out when hot, and working again once things cool off. For about $20, it's worth a try.

 

[DavidL]

I like that idea and local O'Reillys has it in stock. I'll post back with results. Thanx

 

[DavidL]

Part is in, and lo and behold it still runs! Haven't exercised it to see if the real issue is resolved. The old one was stuck a bit in it's hole. Pryed on it gently and it snapped in half but came right out afterward. So, not sure if it was just brittle with age and temp or if it was cracked before, but it sure needed to be replaced. The good news is the sensor was $20 and I had a $20 O'Reilley's coupon...so a gallon of antifreeze (needed to change that anyway as I have no idea how long the old coolant was in there) and fingers crossed.

 

[DavidL]

I ordered a new oil pressure switch which converts from a Make / Break 3 connector sensor to a 2 wire plus a diode. I got the supercedence installation info from JLG today. I put it all in, and noticed the electric fuel pump doesn't run until oil pressure is up. When cranking, this can take 10-15 seconds of crank time to get oil pressure to where the fuel pump will engage...this doesn't seem quite right to me. Any experience on the topic? Does your 600A start right up cold or do you have to crank a while to get fuel pressure? I guess there should always be fuel in the carb from prior start and ideally it would start on that when cold....but I am seeing long crank times. I'll check if the fuel is draining down / boiling away when it sits.

If you were to ask: I have no history on the work done on this rig. The wiring diagram showed a three position oil switch, yet the rig had a two wire. It was only when I tried to order the "right" 3 wire when I realized it superceded to a 2 wire plus diode. Apparently the upgrade had already been done on this unit so it was all kinda wasted effort.

I did let it sit and idle once started until it idle'd up (I think based on engine temp) and drove forward / back a bit to get it to high throttle...so far no cutout post the crank sensor...fingers still crossed!

 

[TheFixer]

I would look at the schematic again. Sometimes they run the pump when the key is in the start position while cranking until oil pressure is up. It depends on how they kill the engine when the oil pressure drops too low.

 

[DavidL]

Well, used the lift for a few hours today and it kept running fine, even when hot. New issue is I shut it down from the platform, and tried to restart...cranked on and off for minutes with no fire. Kept trying and finally it fired and I was able to drop the lift and get out of the cage.
I assume it had oil pressure as it was running well before I shut it down and tried to restart.
Possibly the oil pressure dropped faster as it was hot.
I'll try it again, this time on the ground to do some tests.

The reason I shut it down was it would not get out of Jog mode. The lower cylinder would not drop all the way, though it would go up (slowly). Machine wouldn't drive normal speed (jog speed only). Once it did restart, then all things seemed back to normal.

 

[TheFixer]

You could have a limit switch not working right if it doesn't get out of jog mode. Probably not related to starting, but maybe causing the delay in your bucket swing. Maybe jog mode adds the delay.

 

[DavidL]

Hi TheFixer,
Would restarting the engine reset a limit switch?
What's a way to diagnose which switch to look at closely if the unit doesn't have a scan tool monitoring capability?
I assume once you know which switch to look at, that you would use a idiot light or multimeter to check that the switch opens and closes? Would you do this within the base circuit board cabinet?

The Crawl / Go Fast rotary knob on the cage console (left side) is a little suspect as it feels a little rough and it's fast / slow limit stops aren't aligned with the decal.

 

[DavidL]

I have been using the 600A long enough now to know that the root cause of the engine hot stutter was fixed with the crank position sensor. Thanx OFF for prodding me in the right direction!! That was the major issue with this unit. Now have a hot restart issue that seems to be carb related since it will always restart with a quick shot of ether into the carb (hot or cold).
Once the engine is running, it runs fine now.

 

[TheFixer]

Good news. Maybe fuel pump or line issue. Mine does the same when the fuel is low. Once the pump is primed it starts up, but if it sits for awhile with a lower tank level, it won't start. Not sure if there is a filter in the tank or not that may be clogged. I need to look at one day.

 

[OFF]

There should be a fuel shut-off solenoid (also called the anti-dieseling solenoid) at the base of the carb, just under the float bowl/carb body. Look for 1 wire running to the base of the carb someplace. This little solenoid valve needs power to turn on the fuel flow. Check your power first. If it's getting power at all the right times, then the valve itself could be faulty. It should click when power is applied to it.

 

[DavidL]

Looks like the fuel solenoid and the fuel pump are on the same circuit. Both should get 12+ when starting. (don't think they are). Once oil pressure is up, then they both get 12+ (I think this works and what it runs on).

 

[DavidL]

the fuel pump and the solenoid are getting 8 volts while cranking, 10 when stop cranking and 12.9 when running. Starter is apparently bringing the voltage down. Started right up with a shot of ether...

 

[OFF]

That 8 volts while cranking might not be enough to open the fuel solenoid. Is your battery weak? It should have approx. 12.4 to 12.6 volts at rest and something over 13 when running (under normal conditions). A battery should never fall below 10 volts while cranking (supplying its full rated CCA or CA)

 

[DavidL]

y, I am wondering if there is a high resistance relay / switch / connector dropping the voltage. The starter has good cranking speed so I think the battery is fine. It is also relatively new.
I'll look into it more tomorrow. Thanx.

 

[DavidL]

I ordered a carb rebuild kit as the "accelerator pump" gasket was leaking externally from having been removed too many times. I definitely have a fuel related issue that I believe is making cold and hot starts difficult. It also will occasionally stumble like from running real lean once in a while. I was going to also get the fuel solenoid, but are they sure proud of that one ($150). Mine is moving back and forth so I think it's fine.

 

[DavidL]

Installed new ignition module / map sensor / wiring adapter kit...the old ECU did have a tiny hole that likely was where the old MAP sensor line was connected. Starts up...hopefully it will now run correctly hot and cold.

The rebuild kit for the carb was ordered, then cancelled by the seller as they were out of stock. I'll let that ride until I know if the ECU fixed the problems.