Discussion:
Toyota 42-6FGU25 backfire issue?

Hello FA'ers!

This a first-time post so I'm sorry for anything that shouldn't be done here!! Just let me know...

Anyhow, I have a Toyota 42-6FGU25 with the GM Vortec 6cyl (LPG) that is giving me fits. The truck starts and runs fine under NO load. If I put it in gear or use any throttle angle above idle causes it to backfire about once every three seconds--but only loaded. Idles and revs fine and holds full throttle with no issues (while not moving of course). The backfires are coming from the exhaust. There are no trouble codes stored, spark and compression are good. If ANY load is applied (even tilting forks) I get the backfires. Wires and cap look good, etc. I know that nobody has messed with the engine--I guess it just started one day. As a gas engine enthusiast my guts tell me its either a timing or mixture issue tho I can tell by distributor bolt that it has never been moved/re-timed. Might this be advance? And does anyone know what these should be timed at?

Thank you SO much in advance for your help; you folks are so kind and knowledgeable and I am happy to be in your midst!

Peace,

DoctorRon
  • Posted 31 Aug 2011 23:10
  • Modified 31 Aug 2011 23:11 by poster
  • By DocRon
  • joined 31 Aug'11 - 7 messages
  • Illinois, United States
Ron Damato
King Supply, Inc.
704-360-0087
Showing items 1 - 10 of 10 results.
Spark plug wires shouldn't be any more than about 10-15K ohms depending on lenth. But thats all magical anyway, resistance, (or impedance in this case), alters w/ heat. Do a tune up 3X a year, change the wires once a year, and drop the oil 3-4X a year and you will not have any those dasterly problems.

The fire order is located on the intake manifold, as I remember it might be 1,6,5,4,3,2.
  • Posted 2 Sep 2011 06:44
  • By 7777
  • joined 8 Mar'11 - 649 messages
  • New York, United States
One last thing...

Anyone know the firing order on the Vortec? Just want to check...

DocROn
  • Posted 1 Sep 2011 23:39
  • By DocRon
  • joined 31 Aug'11 - 7 messages
  • Illinois, United States
WOW, that is a GREAT issue that I totally did not think about. Only had a lobe go flat(ter) once on a GM big block with solid lifters and a bad mechanic (me) trying to adjust with feelers and not knowing what the **** I was doing (I was 15...) If exhaust seats are wearing will there be more blowby in the crankcase? Probably not, huh? So used to working on sidevalve (Jeep 6) engines...

Since you know a lot obviously, any idea what to time the GM Vortec at? Right now I'm just guessing but since the 0-10 speed DRAMATICALLY improved I'm guessing its too advanced... ; ')

Also I have some clips a friend came up with that temp. clip to the rockers and re-direct the oil so you can do just this without most of the mess! Lucky me.

Thank you!!!

DocRon
  • Posted 1 Sep 2011 23:34
  • By DocRon
  • joined 31 Aug'11 - 7 messages
  • Illinois, United States
if tune up, firing order, governor, carb, and other peripheral repairs dont help. could be a flat or worn cam lobe. pull the valve covers and watch the travel of each rocker arm as you crank the engine. you can do it running but its kind of messy. i have only seen this happen twice on the toyo product but it used to be very common on the 4.3 gm in other applications. could also be exhaust valve issues "recessed or worn seat" but thats a little harder to check. pease note that my suggestions only apply when you have done or checked all of the usual suspects.
  • Posted 1 Sep 2011 23:28
  • By rick_c
  • joined 30 Jul'09 - 204 messages
  • Texas, United States
technology: (no user serviceable parts inside)
Hello,

I have NO idea why my last post did not show up...

Anyhow, it was just thanking you all and letting you know what I found.

YES, it is the wires. When it was running and I was grounded 3 of them gave me a shock right thru the insulator!! Anyone know if there is an ohm reading or something that "good" wires should return?? I'm just replacing them all, along with the (stock) plugs with 5k hours on them, the cap, and the rotor.

It was NOT backfiring thru the muffler--it was thru the CARB. It blew the filter housing apart!!!

Also needed to fix accelerator switch and lube idle kick-up solenoid. WITHOUT the ignition parts she runs 399% better!!!!

I noticed a tranny leak yesterday also. Does not appear to be a seal. Is this common? If so, anywhere I should look FIRST?

Thank you, thank you, thank you!!! You folks are SO kind and if ever any of you needs any IT advice (that's what I REALLY do) feel free to write. Email is kingsupply AT clear DOT net.

DocRon
  • Posted 1 Sep 2011 23:18
  • By DocRon
  • joined 31 Aug'11 - 7 messages
  • Illinois, United States
Hey,

No, swear to God, its a GM Vortec 6. Perhaps I got the model wrong...there are about 100 here!! I know the truck weighs in at just over 13,500 lbs gross.

Thank you for your info and for helping to pull my head out of my ***!!!

DocRon
  • Posted 1 Sep 2011 23:11
  • By DocRon
  • joined 31 Aug'11 - 7 messages
  • Illinois, United States
When you do a tune up, remember to to replace the spark plug wire set. Sometimes they break internally and allow for idle and back fire under load. Connections to the coil and/or a bad coil if its a 181 4cyl. GM, if a 4Y toyota engine, the entire dist has to be exchanged.
Always start simple.
  • Posted 1 Sep 2011 21:03
  • By 7777
  • joined 8 Mar'11 - 649 messages
  • New York, United States
First if this is a 5000lb capicaty truck as stated then it should only have a 4 cylinder engine, Toyota did not put 6 cyl in any trucks smaller than a 8000lb capicaty truck (ie 6fgu40). That stated it can either have the gm3.0 or the toyota 4y depending on the year of manufacture. Regardless of engine as Kevin stated I would start with the simple things like a tune up. From there I would check the governor, then the distributor gear for wear, then the timing belt/chain(depending on engine) for slipping or wear. I have seen all of these items cause issues like this. I would also check the transmission pressures as a problem in trans may be causing excessive strain on the motor. hope this helps.
  • Posted 1 Sep 2011 08:36
  • By certifiedtech
  • joined 5 Aug'10 - 39 messages
  • Ohio, United States
Do you have an email???
  • Posted 1 Sep 2011 06:57
  • By kevin_t
  • joined 2 Dec'10 - 1,301 messages
  • Pennsylvania, United States
Try something simple first like changing the plugs,had a hyster a few weeks ago that would pop every so often changed the plugs and cleared it tight up.
  • Posted 1 Sep 2011 06:55
  • By kevin_t
  • joined 2 Dec'10 - 1,301 messages
  • Pennsylvania, United States

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