Toyota 42-4FGC25:
Engine stumbles, chokes, died, now won't start

Hi all. My friend has a Toyota 42-4fgc25 forklift with a Toyota 5r engine. The engine utilizes an Aisin carburetor and Aisin convertor/regulator. Recently he complained that it started to run rough and smelled heavily of propane. It would only run at half to full throttle otherwise it would die. It smelled like it was running super rich. After a few more days it wouldn't start at all regardless of what we did. Long story short after countless hours of troubleshooting we determined that we had a fuel supply issue. If you use an external propane supply (tiger torch forcing propane into the carb) with the carburetor intake pipe removed from the top of the carb the engine would run and operate properly.

As such we replaced the Aisin convertor/regulator assembly with a good used unit from eBay. After we installed the "new" unit we started the engine but only if the throttle pedal was at full throttle. It would run ok but die if it was allowed to "idle". We ran the engine at part throttle until it warmed then adjusted the idle. After the engine warmed it would idle fine and run properly. There seemed to be a lag in the throttle response but otherwise it ran as per usual.

The ambient temperature during this has been anywhere from 35*f to 60*f. Fast forward a few days and now it won't run again. It coughs but will not fire. It tries to start but will sometimes backfire and then just die. It's giving identical symptoms as the original issue. It sometimes seems like it will almost fire and other times just cranks. To date we have replaced the coil with an aftermarket coil, replaced all spark plugs, cleaned the distributor cap( non-points style ignition with magnetic pick up with vacuum advance), replaced the battery and tested the following components.

More to come...
  • Posted 5 Mar 2018 07:07
  • Modified 5 Mar 2018 07:25 by poster
  • Discussion started by CanadianBacon
  • British Columbia, Canada
Toyota 42-4FGC25
Showing items 1 - 6 of 6 results.
Being one of the original Toyota Industrial Truck employees, I have had to troubleshoot these many times. Take apart the LP solenoid valve and see if there is any rusty looking material in it. There is a filter in it. The bottom of the regulator has a drain plug. Open it and drain any residual oil from it. If nothing drains, it probably means it is gummed up. The secondary valve pressure should be 3.5~5.0 lbs/sq.in. The screw on the top of the regulator is an idle fuel mixture.
  • Posted 7 Jan 2021 16:09
  • Reply by charlie_j
  • California, United States
Take time to do the job right the first time, or you will have to make time to do the job over
Thanks for the update!
  • Posted 25 Jun 2018 05:50
  • Reply by forktime
  • Alberta, Canada
FYI, for anyone experiencing issues like this going forward here is what was found.

The regulator migh have initially been faulty causing an over fuel, but the new regulator was fine. We were able to start the engine using an external propane supply, then keep the engine running using the forklifts own fuel system.

The only possible items left were the carb and governor. We ended up finding that once warm, the engine start d easily every time. We took apart the carb, cleaned all the passages, cleaned the idle screw and seat, cleaned what I would describe as the "accelerator pump" mechanism, then reassembled. Cold the engine fired up if you depressed the throttle. Seems to run fine now. I believe the carb was just gummed up. Hope this helps someone else if they experience the same issue.
  • Posted 9 Mar 2018 18:03
  • Reply by CanadianBacon
  • British Columbia, Canada
Toyota 42-4FGC25
Mrfixit,

Thanks so much for the suggestion. I have thought myself perhaps the supposedly new regulator was faulty. I contacted the seller and he only used it for a short period of time before finding out that he had a dead hole on his engine thus the reason for selling the regulator.

With that said I don't discredit your suggestion. When we purchased the new used regulator we also bought a rebuild kit. I will reseal the original regulator assembly and see if that changes anything.

Regarding your test suggestion I will submit some more info you may find valuable. When we had issues with the original regulator assembly we used a tiger torch with a flow valve so we could control the flow of propane into the carburetor. We were able to run the engine with the forklift propane tank valve off. Also on this forklift another mechanic wired in a toggle switch for the electric solenoid lock off valve. Without the toggle on the solenoid will never engage.

We tested the engine and with the external supply it ran, so we decided to start it with the external supply then turn the forklift tank valve open to see if the engine would stay running. When we tried this it wouldn't which is when we came to the conclusion that it was a fuel supply problem. We found that when held about 2" above the carburetor the engine would run, any higher and it would die. This all seemed normal and pointed us to the direction that there is no propane getting to the carburetor, not too much propane.

My friend being who he is went to a local forklift shop and they sold him an impco regulator and convertor two pot system. He installed the system and we again attempted to start the forklift with no luck. We again tried the external propane supply using the tiger torch and the engine ran again. From my research the impco system uses some sort of a membrane vacuum operated valve on the carburetor to allow a vacuum to be pulled to start the flow of propane vapour from the convertor. We tested the operation of the new convertor and regulator and both seemed to work properly. The vacuum valve worked, we tested to verify good propane out of the regulator by disconnecting it from the convertor and using a vacuum pump to open the vacuum valve on the regulator. After this we used gentle shop air on the convertor while drawing a vacuum on the propane outlet port and the convertor seemed to work as designed.

At this point I told my friend you can't start mixing and matching systems, you need to use all the components from one system. I figured that we must have a missing piece that is required for the impco system not present with the current Aisin system. This is when we decided to purchase the new used Aisin convertor/regulator that was the identical model to the first. When we first installed the new used unit, it started and ran fine for an entire day. We stopped and started the engine and had a stable idle and everything seemed good.

It was over the next few days that I later found out the engine stopped starting again. It is possible that there could be an issue with the new used regulator so that isn't bad to check but I felt I would explain some more background testing.

I am going to reseal and test the original regulator assembly to see if that fixes the issue. I also read that some forklift systems route power through an oil pressure switch to the fuel lock solenoid. I will test to see if this forklift is the same. Perhaps the solenoid is only working intermittently which is why sometimes you hear propane flow after or during cranking and sometimes you don't. I will post back with the results.

Thanks again for the suggestion Mrfixit, it's appreciated.
  • Posted 6 Mar 2018 14:55
  • Reply by CanadianBacon
  • British Columbia, Canada
Toyota 42-4FGC25
I would shut the propane tank off, drain all the propane from the lines, squirt a little gasoline in the carb, and see if it fires up. If so, I bet the used regulator is no good. When regulators go bad they usually over pressurize and once too much propane starts flowing the motor will refuse to start.
  • Posted 5 Mar 2018 22:11
  • Reply by mrfixit
  • New York, United States
The electric fuel lock-off solenoid functions properly and isn't restricted. It appears to be getting a signal from perhaps the alternator as it will only energize when the engine rotates. It does not energize with the key switch on start or run. There is liquid propane to the convertor/regulator. I have not measured the pressure as I don't know exactly what it should be. We have verified base engine timing is correct by removing e valve cover, rotating to the tdc mark on the crank pulley and verifying that the distributor is aligned with #1 cylinder with #1 valves both "loose". When it was running we rotated the distributor advanced/retarded and it seems to be adjusted properly based on how it idled and ran although I have no advance specification. I read about 8* advanced but not sure this is correct. The crank pulley has three timing marks and I have no explanation of which means what. We are going off of the largest notch which if I recall is in the middle. We have used two fresh full tanks of propane and they are verified good as they work on his other forklift. We used a spark tester and all plugs are firing. We cleaned out the intake manifold as there was a tremendous buildup of crap, mostly black crud that was gooey which I assume is carbon and unburnt propane. The coolant system is topped up and operating properly. The regulator/converter does not ice up when running. Vacuum during cranking bounces around 5in mercury. I never did check running vacuum. I hooked up a vacuum pump to both the old and new convertor and the vacuum lock off works. I tested the vacuum advance on the distributor and it didn't seem to work, but I can hardly see is as the issue as we did have it running well with this same distributor. We tested compression and all cylinders are at 170psi. Cranking speed is good and we have the battery on a charger when testing and cranking. The oil pressure dummy warning lamp goes off when he engine has been started. There is no Murphy switch that I can see. We have cleaned out the intake pipe that bolts to the carburetor and replaced most of the vacuum hoses. We have tried to run without the air filter to see if it was clogged but that made no difference.

There appears to be some sort of governor under the carb but I dont know how it works. We tried to manipulate it by hand with the cover off but it didn't change anything. It bounces around as it uses a vacuum hose to ported vacuum rig above the carburetor inlet. I assume it regulates intake air based on vacuum but have no information to confirm. There is an adjustment for it but it is safety wired and we have not adjusted it. We have not touched the throttle plate stop but have tried with the throttle at several positions and cleaned the entire carburetor with brake clean. When the throttle is at maximum there is some sort of "accelerator pump" on the carburetor but I have no idea it's use or operation. There is a solenoid on the side of the carb that does seem to work, but I don't know it's significance. We tested it and it energizes, and we tried jumping it with direct power but it solved nothing. There is a plate under the carburetor that vents to what appears to be a crankcase breather bolted to the side of the engine block. I plugged the hose to see If we had a vacuum leak but that solved nothing. I am assuming this is an emissions device but can't confirm. I removed the check valve inline with the hose from the carb to the piece on the block and cleaned it with brake clean. It moves freely when you jiggle it as it should in a regular gasoline engine. I am in need of someone who Is experienced enough to have dealt with this type of lift. It seems the issue is that there might not be significant enough vacuum to pull in the propane perhaps. Sometimes you can hear the propane vapor escaping into the carburetor when you crank, and other times you hear nothing. I am a heavy duty Diesel engine mechanic by trade. If someone has specs on pressures I have all the tooling needed to check. Also the convertor has a scare to adjust the distance that one of the vacuum operated membranes moves but I have no information on its proper adjustment. Any and all help is appreciated guys!

P.S.

I have tried to determine how to get some photos uploaded but didn't find any information. I have pictures of the forklift, the carburetor, the regulator/convertor and the data plate. The serial number is 404FGC25-20778.

Ben
  • Posted 5 Mar 2018 07:26
  • Modified 5 Mar 2018 07:33 by poster
  • Reply by CanadianBacon
  • British Columbia, Canada
Toyota 42-4FGC25

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