Hi all, I just got my truck out and about again after our prolonged winter here in Michigan, and I ran into an issue on the highway. Let me tell you the story to hopefully answer as many detail questions as I can. First I had to run an errand, so I started the truck on this balmy, 68 degree day and drove down main street (Woodward) about 4 miles in steadily moving traffic to my destination and parked the old girl. No issues. I've got a temp gauge in either head, and both looked good. Left there and headed to work, about 15 miles away, mostly interstate. I've got a 3.54 gear in my Columbia and so can maintain 70 pretty easy, and while passing a tractor-trailer I started to lose power. Stereotypical 'lost fuel pressure' type of bucking, and I was slowing pretty fast, so pulled off the highway, luckily there was an exit, and shifted out of OD and let it coast for a second while I listened to make sure it wasn't running. It wasn't. So I put it in 3rd and tried to start it that way, and what do you know, off she went! Ran fine like that for a while on the service road, so I got back on the highway to see if I could replicate it. Sure enough, it started bucking again after a little while running 70/75. I was able to finesse the go-pedal enough that it didn't stall, but it continued to buck lightly for a bit until it smoothed out and got me to work fine, no other issues. Thoughts? Other pertinent info: mechanical fuel pump, 94 carb (rebuilt), new fuel lines all the way back, and a new fuel tank. Checked all temps when I got out with a laser thermometer and nothing was over 210 (which was right on the heads, 190 on the outlet hose). The carb baseplate was a bit high to me at 160, but the bowl and throttle body were well below that, about 130.
I replaced both before I put it up for the winter! Not saying one - or both - can't be bad again, but it sure makes troubleshooting a bitch.
That's my first thought. Wonder if the fuel pump is going TU and not up to supplying enough fuel to keep the float bowl full? Bowl runs low, pump supplies enough fuel to cover bottom of bowl only, engine burns that up, pump supplies another partial shot of fuel....repeat = surge, surge, surge.......?
Sounds like a fuel starvation problem. Can you disconnect the fuel line at the tank and check for blockage? Disconnect the lines and blow them out too. If that all checks out I'd look at the fuel pump.
Fuel filter? Leak on the suction side of the pump/tank... on the suction side a leak will allow air into the system and the fuel doesn't necessarily leak onto the ground. Vapor lock could be an issue but it is early in the year for vapor lock unless you managed to get one of the fuel lines near the exhaust.
Check the flex line between the firewall and pump. I've had them de-laminate and the inner rubber liner collapse, from pump suction. After the suction ceased and allowed the liner to regain shape it ran normally until on the freeway again.
Interesting thoughts fellas, thanks. I hadn't thought about possible rubber de-lamination, or loose connections letting in air, I'll look at those first because they're easiest. While I'm at it, blowing the lines out is easy enough. @Doublepumper - the fuel pump fills the bowl just fine, which isn't to say it does at speed... I did lengthen the pushrod a little last year as I found it was mushroomed, so it should have plenty of stroke to do its job. Worth checking though, I've got another few on the shelf.
Typically your fuel demand will be the highest on the highway(maximum load). What is your fuel pressure while running down the freeway? Does fuel pressure fall off when it starts running poorly? If not, it’s not a delivery issue. Sent from my iPhone using The H.A.M.B. mobile app
Good point. I don't have a fuel pressure gauge permanently installed, but it would be worth putting one in to troubleshoot.
I too had this problem. Even with new hose. So i had one made with movable ends. Check your fuel pressure out of the pump. Should be around 4 lbs. next time it does it, turn the engine off and take a look at your float bowl.
Next time it happens, turn off the key and pull over. Check to see if the float bowl is empty so you know if you're dealing with fuel or ignition.
Somewhere in posts 4,5 & 6 is the answer. First off, what did you change in the complete fuel system from cap to carb since last year? If you let up on the gas for a few seconds and it went again it means that the float bowl was running out of gas at speed. Any fitting changes? Some brass fittings that have the right threads are made for Natural gas applications and have a tiny orifice inside a fitting with larger threads. I had that happen on a 327 with 2 4's that I bought Truck idled and cruised great but ran out of gas on a hard push. The delaminated hose suggestion is a good one as we are seeing a lot of that with today's gas eating up the rubber. It probably wouldn't cost very much to change out the hoses for the newer style that handle the new gas. A fuel line that got kinked when someone bent it to fit. Steel or rubber. A non vented gas cap (or one not venting right) will let you run a ways before the engine stalls but you have to wait until the vacuum created in the tank stabilizes before the pump will pull gas again until the vacuum in the tank overcomes the pump again. Shiny new gas cap by chance?
I myself find the answer in posts 11 and 12 . Often we see " I had that symptom and did this" as opposed to easy, logical diagnosis. This old stuff is pretty dang easy to diagnose!
We had the same surge issue that you are describing on a 53 Buick with a 401 nailhead. It ran fine untila certain speed and we could see the A/F ratio go to 18 or 20 and the engine would stumble and die. He had a kinked and too soft rubber line going into the fuel pump, and it collapsed at a certain engine speed. He had been chasing his tail for some time on this one..
Thanks for the ideas everyone. The truck is in the upholstery shop for a couple weeks, so updates will take a minute, but I'll keep you posted on what I find!
is this the first time you have driven this configuration at this speed? have you made any changes since the last time it ran trouble free at those speeds? we had an old tractor that we needed to put a new fuel filler cap on. it would run fine with no load on it and it would run for about for 30-45 min (depending on fuel level) with a load and then die, we would wait about 10 min and it would fire up and run for another 30min or so. The cap we put on didn't vent well enough and we would get just enough negative pressure in the tank to keep the fuel from flowing enough to meet the demand with a load on it. in our case, in the middle of a silent field you could here it suck air when you twisted the fuel cap off.. might be hard to hear on the side of a busy highway. Chappy
Hey all, recently got the truck back from the upholsterer, and it looks good! But I still have a problem with getting it running right, here's where I'm at: Drove it around a bit after getting it back and realized it wasn't right, so I started looking. Found that with the choke pulled I could finally set the idle and it would run ok, but still had some surging (RPM up, RPM down, then level for a bit, up-down, repeat). This told me I had a vacuum problem, so I started checking all the usual places - no luck. Ran into this thread where the OP mentioned he put his hand over the oil vent and had negative pressure, so I thought "wow, that's something I haven't tried" and sure enough, same issue. Now the intake is off, the heads are off, and so far all I've found is at least one intake valve that needs to be lapped and maybe have a full valve job done... good thing I bought myself a valve grinder a couple months ago. I still don't feel like that's the real issue though, so I'm hoping someone's got more thoughts. Oh! Now that it's apart, I've found I've definitely got a fairly low-time engine, but one that's sat in that condition for quite a while. It has later style straight valves instead of tulips, and adjustable lifters with domed pistons. I'm a bit happy to find those bits if I'm honest.
Did the intake guides have the rubber seals on them? Were they all pretty tight in their bores? If so, maybe the intake gasket was leaking? On the stumbling issue, I had a similar thing happen on my '52 flathead. The fuel pump would build pressure at idle, but run out of volume at cruise. Took a while to find that one.
So far I've only pulled one valve out - the one I could definitely tell needed to be lapped. It didn't have any seal on it, but was fairly tight... I'm thinking the combination of valve blow-by and missing seals could be my culprit. To be honest, until doing some research I wasn't even sure it needed seals.
A lot of rebuilders never used them on solid guides as it makes them much harder to install and/or remove. Some used goop instead. Depends on the fit of the guide to block
Very interesting... There was nothing on any of the ones I've gotten out so far (all intake), so I'm not expecting any on the exhaust either.
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