Bucking bronco

I'm hoping someone out there can come up with an answer to my recent problem.
Last Friday I was on my way to our local cruise night. Going down the road at 35 mph, my truck suddenly started to lose power and started to buck wildly. I was in 4th gear at the time. I put in the clutch and pulled over to the side of the road. Stopped the truck, but never turned the motor off. Started to drive again and about 5 minutes later it started to lose power and buck again. I thought maybe I miscalculated my fuel level, (my gauge doesn't work, so I go by miles driven) and pulled into a gas station and put in $20 worth. It then ran fine all the way to the cruise parking lot. Upon leaving a few hours later, it ran perfect until I got 2 blocks from my home and it started the same bucking routine again. When I got home, I let truck idle in front of the house and it idled absolutely perfect, not a single sputter.
Electrical problem??? Fuel delivery problem???
Thank you.

Blow some compressed air into the fuel tank through the pickup hose. Just a guess but I had similar issues with a clogged pick up sock.

Actually sounds more like the clutch is slipping. You been riding it? Does it smell burn't.

Never ever ride the clutch kwai. Driven manual tranny's all my life. No burnt smell either. The hardest work my clutch goes through is during parades. Clutch actually went through my mind also, but for the most part, the truck runs fine as I go through the gears.

I can give that a try Barrman. Truth is though, when I had the tank off and boiled and lined, I never remember seeing any type of sock over the pick up end of the sending unit.
Would you still give the air blowing a shot?

Is an electrical problem of some sort even a thought at this point? Just seems strange that the truck can move along so well, and just like that she starts bogging and bucking.
The perfectly steady idle is what has me confused.

Does the engine continue to miss (creating the bucking) when you push in the clutch or just under load with the clutch engaged.


another way to eliminate the fuel delivery is to strap a 5 gal gas can to the bumper. You can run the fuel line to the drivers side of the engine where the frame hardline transitions to a rubber hose going to the hardline on the engine block.

You can check the clutch/flywheel by dropping the dust cover, you don't have to remove the tranny.

The losing power first has me thinking fuel as well. I don't suspect the clutch by your description. I would think if it were clutch it would start slipping and the engine would start revving but you didn't mention that.

And electrical doesn't seem likely because the problem isn't happening at idle.

The engine never quits running. When the bucking starts to happen, I put in the clutch, come to a stop, and then start out again. It moves fine for a short while and then starts the bucking again. This probably all sounds strange, but this just started happening last Friday evening. Never had a problem like this before. Whats got me even more confused, is the fact that at idle, the motor runs as smooth as can be. On Friday, the three times this happened were all while the truck was in 4th gear and moving forward at a steady speed.

The smooth idle says "fuel starving" to me: the fuel pump is delivering sufficient fuel to maintain a good idle, but is unable to deliver enough for the engine to perform under load. The delay in the onset of the bucking seems to me like the time needed to empty the float bowl and start starving for fuel.

HOWEVER, one question: you don't have some sort of electronic ignition, do you? This also sounds like what happened when the ignition module on my M886 went TU: it would idle, but wouldn't perform under load, for some reason.

I do not have an electronic ignition. I did purchase a Pertronix ignitor kit, but never installed it because the truck runs so well.
Any suggestion Nailhead, on what to do, should it be a fuel starving situation?
24v inline fuel pump??? Bad diaphram in pump?
I only use Mobil or Shell gas in the tank and the tank is in perfect shape.
Could the sending unit be bad? This all sounds unreasonable because the truck idles so well.

Somebody put marbles in your gas tank? .. don't know that that actually works, always heard that though. Does sound like a fuel problem though.

A hole in your pickup tube allowing air in below a certain level? ... $20 worth should have kept you above that level for a while though.
Weak pump or stuck float...? although I've never heard of a float sticking in a down position.. you could be the lucky first.

Wish i knew for sure...years ago a member had a problem like this after he changed his points...seems the leaf spring of the points was not installed right...didnt put tension on the points to close them...rough running happened in high gear as rpms rose...motor was under enough load for it to show up...
I realixze you didnt touch the points but could the leaf spring in there have loosened up or broken?
Just my wag...

You know Jon, I will check that out. My rough running takes place as I'm cruising along in 4th gear at a steady speed of about 35-37 mph. I'm a little confused as to why it wouldn't happen as your rpm's increase as you reach your shift points in the first 3 gears though.
My thoughts are, if it was a fuel pump/supply issue, why would the truck run so smoothly at idle? I really don't suspect a bad coil issue either. My electronics knowledge is limited to fairly basic stuff, but a loose/broken points spring should be easy to detect I would think. Although, if the spring was loose or broken, wouldn't this effect the firing order through all gears?
Now that I think about it, I believe I heard one or two very small backfires while this was happening last Friday evening. The backfires came while the truck was bucking and loosing throttle power. Any clue there?

I don't remember if you have checked the coil in the past. I can tell you that my "No Restart When Hot" was one of those bloody Chinese chrome coils. I installed a vintage, used coil and that problem went away. Have you been inside of the ignitor? Wires OK, etc.? A condenser could also cause issues.

But bad points could do exactly what you describe. And, you are using very little fuel when idling so if it is a carburetor or pump or clogged filter thing, that could cause it too.

Do you have a clear fuel filter? I put one on my truck temporarily so that I could monitor the fuel. I have a new steel one but have been too lazy to change it. (I could say, "If it ain't broke, don't fix it" but I know it is laziness....)

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Originally Posted by militarypotts

You know Jon, I will check that out. My rough running takes place as I'm cruising along in 4th gear at a steady speed of about 35-37 mph. I'm a little confused as to why it wouldn't happen as your rpm's increase as you reach your shift points in the first 3 gears though.
My thoughts are, if it was a fuel pump/supply issue, why would the truck run so smoothly at idle?

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Had a similar discussion on an Isuzu FB group the other night. A member had the opposite problem, His truck ran great at speed but was dumping fuel down the throat and dying if it idled for a little bit. This was after he put a new carb on. I haven't heard back from him yet, but the top two theories was either his fuel pump was putting out too much pressure for that carb or the float was adjusted a little too high. Figuring that if the valve was staying open just a little too long it might not be a problem while he was running at speed, but when it sat idling and not using as much fuel then it would fill the bowl up and dump.

Maybe I just had carb problems on my mind when I read your post, but it sounded like a starvation problem... but just a slight starvation... more like a diet..
Like you're getting enough fuel to idle all day long and run through the gears but after running at speed for a bit you run through the available fuel faster than what's being supplied.
That's why I was thinking weak pump or a float problem. The fact that you can pull over when it starts (letting the bowl refill) and then get back on the road for a bit reinforces it a bit.

All of these answers seem to make sense, but I'll start with the ignitor. All seems weird to me though, that it was running perfect up until last Friday evening. I know that sounds dumb, because issues can happen at any time, but their wasn't a sign of this at all, in any way. I guess one thing I should do is get the truck back out on the road and see if it does it again. Trouble is, when things like this happen, I get a little gun shy about taking her back out.

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Originally Posted by militarypotts

...Trouble is, when things like this happen, I get a little gun shy about taking her back out.

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Boy, do I understand this...

Me too...Hope you have an easy solution!

Keep us posted!

Will do kyken. Unfortunately, I haven't had the time or energy to start looking at it. Maybe this week I'll start with the points spring. I really think I need to bite the bullet and take her out on the road again to see if it starts happening again though.

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Originally Posted by militarypotts

I really think I need to bite the bullet and take her out on the road again to see if it starts happening again though.

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Test drives are so exhilarating! I always make sure I'm wearing comfy shoes, have my phone, an extra pack of smokes and a full thermos with me.

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Originally Posted by Von

Test drives are so exhilarating! I always make sure I'm wearing comfy shoes, have my phone, an extra pack of smokes and a full thermos with me.

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Or instead of the Thermos, a six-pack...

My problem is, as a military collector, I have my truck filled with items in the cab and also the bed. I have two rifles in the cab, not to mention a PR25 radio that actually came out of Vietnam. Not to mention the M72 Law Rocket Launcher hanging from the top bow. All this and lots more, make it so I can never leave my truck unattended. I have to stay put and wait for the flat bed to come get it. I have to be honest with you guys and tell you that sometimes owning and drivings these things can be a little on the stressful side. Anybody else agree with that?

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Originally Posted by militarypotts

My problem is, as a military collector,...
All this and lots more, make it so I can never leave my truck unattended...
...Anybody else agree with that?

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Could not agree more. For that matter, I removed the key-type ignition switch which was installed by the previous owner. My truck is 100% stock. All you have to do is flip the ignition switch, step on the starter button and off you go.

Do you think I can walk away from my truck? And, I don't have all of those neat extras that you have.

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Originally Posted by militarypotts

My problem is, as a military collector, I have my truck filled with items in the cab and also the bed. I have two rifles in the cab, not to mention a PR25 radio that actually came out of Vietnam. Not to mention the M72 Law Rocket Launcher hanging from the top bow. All this and lots more, make it so I can never leave my truck unattended. I have to stay put and wait for the flat bed to come get it. I have to be honest with you guys and tell you that sometimes owning and drivings these things can be a little on the stressful side. Anybody else agree with that?

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Yikes! Yeah, wait for a buddy with a tow strap to follow you around.

When I got my truck stuck at the lake last year.. I pulled 3 out before I succumbed to the bog... I ended up leaving it out there overnight. I do have a keyed switch, not that the silly thing was going anywhere without an airlift.

She's a great truck, but not what you'd call a showpiece. I was more worried about my chains being stolen.

When you get to it and get it figured out let us know.

Took apart the ignitor yesterday. Checked for loose or broken points spring, loose wires, rotor, etc. All appeared to be ok. Cleaned up face of rotor contact, although it all looked good even before I did. Anyway, I put it all back together and fired up the truck. She started right away, and again idled smoothly and trouble free. Once again, I guess I have to man up and get this thing on the road in 4th gear at a steady 35 to 38 mph and see is she starts to loose power and start bucking. I'll keep everyone informed when I do.

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Originally Posted by militarypotts

I have to be honest with you guys and tell you that sometimes owning and drivings these things can be a little on the stressful side. Anybody else agree with that?

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After three flat tires on the same wheel, the last without any apparent cause 170 miles from home, I know EXACTLY what you're talking about.

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Originally Posted by militarypotts

I have to be honest with you guys and tell you that sometimes owning and drivings these things can be a little on the stressful side. Anybody else agree with that?

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Yep, me too. My first drive after the restoration, including the electrical, ended in less the 4 miles when she quit. China coil but I never got over the anxiety of any trips.

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Originally Posted by rboltz

Yep, me too. My first drive after the restoration, including the electrical, ended in less the 4 miles when she quit. China coil but I never got over the anxiety of any trips.

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I am with Ron. China coil was the problem with my truck but at the time, I did not know. Now, with a stainless gas tank, original radiator with a new core in it, all is OK. But... I would not drive it from here to California. I guess what worries me is the gearboxes. I have not replaced seals yet. Yes, gear oil is cheap. But...

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Originally Posted by Don Cavey

.....All you have to do is flip the ignition switch, step on the starter button and off you go.

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Don,
While I would be worried like you, there probably aren't many thieves that can drive a manual shift, let alone smart enough to know what the foot switch is for.

I got a call recently from two co-workers (46, and 54 years old) asking me how to start a Dodge M37 they were looking at.
They figured out the ON switch but were baffled by what to do next, as neither had ever seen a 'stomp starter' before.

Thanks guys for not making me feel goofy about being gun shy after the truck has some sort of problem. With that being said, I was just about to give the next chapter in my bucking bronco post.
So, tonight (Friday) is our local cruise night about 4 miles from home. I decided to take the chance and bring the truck out. She started literally on the first push of the floor starter.
I thought, good sign! WRONG!!! I got to the first stop light intersection, in the left turn lane waiting to turn left and BAM!!! the engine just quietly stops running. 5:00pm Friday rush hour. People blowing their horns, unable to make a left hand turn because I was blocking the lane. Police come up behind me and turn on their lights, I have my hood up and emergency flashers going. Police call a local tow company they use and tell them to send out a flatbed. In the meantime, the police ask me if they could take pictures of the truck, because nobody will believe them down at the station. I tell the officer it's ok with me if he wants to take pictures, but most of the police force know my truck from my towns local Saturday night cruise. While I'm waiting for the flatbed, a young man in a Dodge Ram Diesel pulls up and says I'll pull you out of here. I have a tow strap in the truck, but the police said it would be too dangerous in all the traffic. The young man was a Marine just wanting to help an old army guy and his army truck. Tow truck shows up in about 20 minutes, loads it up on his bed and brings my truck about 6 blocks back home. I drive it down his truck bed and into my garage. She's safe at home again.
When all is said and done, I now believe the freaking Chinese coil I put in 2 or 3 years ago puked on me. The motor turns over perfectly and you can smell raw gas at the exhaust tip. Doesn't seem to be getting a spark in my opinion. Anyone agree with me that it sounds like a bad coil? AGAIN!!! I do have a brand new unused Chinese coil in the garage. I guess I have to at least try it. Do you guys feel the same way?

I apologize for the long rambling post above, but I just had to unload on somebody. Hope you guys bear with me on this one.

My advice? NO CHINESE COIL!!!

I got a used coil, $60 and that literally fixed the problem. The M-37 coil should be the same and maybe a 6-cylinder deuce and a half gasser too.

Not the first time a Chinese coil is at he center of a problem.

Consider this. Originally, our trucks had a ballast resistor under the coil. Later ignitors had a coil that did not need a ballast. Could it be that the Chinese coil needs a ballast? If so, it may be overheating and going open? Just a thought.

All the new coils have a build-in resistor. Coil failures are not that uncommon, chinese-made or not. I would replace the coil with what is on hand and then buy 2 new coils for spares. Keep one in the truck.

I am very skeptical of any new coils. Surely, there is a vintage 6-cylinder coil out there somewhere.

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Originally Posted by Don Cavey

Surely, there is a vintage 6-cylinder coil out there somewhere.

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I see them at the truck events however no way to test them so I'm always afraid to stock up. Prices are usually in the $40-$50 range.

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Originally Posted by kwai

Coil failures are not that uncommon, chinese-made or not. I would replace the coil with what is on hand and then buy 2 new coils for spares. Keep one in the truck.

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Lots of documented failures with the china coils. Not so much with US made ones. Yes, you could keep a supply on-hand but why change china coils every 5-10 miles? When I had access to a megger we tested coils. The china ones broke down fast.

I understand the New Star ones are now Korean or Malaysian, not sure which, and are oil filled again. I have no experience with these so not sure if better or not. The story that New Star stopped importing the china coils tells me alot.

I just finished pulling out the coil in my truck. I have written on the side of it, 7/29/15. That makes the coil 23 months old, as far as being installed in the truck. Is their anyway to test it when it's out of the truck? I found another coil in the original box and on the box it's marked "New". Don't know whether to put this one in, or not. I did order another one, but once again it's from China.

You know Don, I'm skeptical also, but what choice do we really have. The seller I bought the new one from called me to discuss the coil and it coming from China. He pretty much said the same thing I just said, but also added, that if the ignition switch was left on, any coil would burn up, China or otherwise. He probably does have a point there. Has anyone ever figured out why the coils from China burn out so quickly? Poor materials would be my guess, but I don't really know for sure.

One other thing I forgot to mention, after removing my coil I noticed maybe 1 or 2 drops of oil in the coil housing inside the ignitor, is this normal? Very minimal, but none the less was there. Also, I must have one of the later ignitor's, because their isn't room at the bottom for a ballast it appears. The coils are short to begin with, and still come to the top of the housing.