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jimbro · 05-17-2016, 10:49 PM

Hi Everyone,

Ive tried to find some related threads to the ongoing issue I am having with my MH engine but to no avail.

I have had absolutely no luck in resolving a miss in the engine when it s under load and fully warm (205-210). The problem occurs when I start up a small or large grade or accelerate to gain more speed after 45 mph.
My repairs to date include; new plugs, wires, dist cap, coil pack, rotor, module, fuel pump, both fuel filters and rebuilt the carb.
I have ohm tested the pick up coil and pressure tested the fuel pressure which is 7lbs after replacing pump.
I also have made sure the vacuum advance if free and seems to be working fine. I did find out I had a non vented fuel cap when it should be vented but the problem still happens even with the cap removed. The problem is very rare when the tank is full and when I reach approx 1/3 of a tank it starts to act up. I ran my camera into the fuel tank and it looks good. I drained the fuel and very little debris in fuel, just a few grains of sand.
What could I be missing?

Frustrating to say the least.

Jim

Hans 34.5 · 05-17-2016, 11:29 PM

Hello
My 34.5 has a electric fuel pump in the back next to the fuel tank 80 Gal does yours have one. I also see it has a 1/2 fuel line that runs on the outside of the frame and turns into the frame rail and goes back to the tank ends up 3/8s. I'm currently changing all the rubber fuel lines and the ones next to the gray tank are a pain in butt.

Hans 34.5 · 05-17-2016, 11:42 PM

Someone could have hooked up the engine gas line to the generator gas line that has a shorter tube in the tank.

J. Morgan · 05-18-2016, 12:12 AM

That is a long draw for a fuel pump, and seeing as the problem is associated with fuel level, I would recommend that if your RV does not have a helper fuel pump, install one, or if it does have one make sure it works.

Superat stultitia.

bkahler · 05-18-2016, 04:52 AM

Airstream (or GM) installed this Carter pump as a booster between the fuel tank and the engine mounted fuel pump. On the 345s it is typically mounted just in front of the fuel tank. On my old 310 it was mounted just rear of the entry steps.

Like others have said, if you don't have the booster pump you really should buy one of these and install it.

Brad

jimbro · 05-18-2016, 06:42 AM

Quote:

Originally Posted by bkahler

Airstream (or GM) installed this Carter pump as a booster between the fuel tank and the engine mounted fuel pump. On the 345s it is typically mounted just in front of the fuel tank. On my old 310 it was mounted just rear of the entry steps.

Like others have said, if you don't have the booster pump you really should buy one of these and install it.

Brad

Ive looked for one around the tank with no luck. I will try the near the step location.
Thanks

jimbro · 05-18-2016, 06:44 AM

Quote:

Originally Posted by Hans 34.5

Someone could have hooked up the engine gas line to the generator gas line that has a shorter tube in the tank.

I thought of that but I can run the tank down to just about empty and my understanding is the generator lines usually terminate around 1/4 tank (full)

tevake · 05-18-2016, 06:49 AM

Another thought occurred to me about your missing during hard accelerating periods. Could you secondary jets in the carb be clogged or partially blocked.

Still fuel supply, just a bit farther down the line.

Sounds like you are doing good things for the good running of the engine.

Keep at it, you will find the issue soon.

Cheers Richard

compubmw · 05-18-2016, 12:47 PM

Remember, your gas is coming from the rear of your motorhome. On mine, the mechanical pump gave out and in addition to replacing it, I also added an inline electric pump because my 86 325 didn't have one or the provisions for one.

Now, I run smoother than ever...

bobmiller1 · 05-18-2016, 03:28 PM

Being that the problem is "RARE" but still happens when the tank is full may lead to not a fuel problem but rather an ignition problem...although you tested the pick-up coil...for the price did you just consider replacing it ??? You may want to try driving it with the vacuum disconnected from the vacuum advance...although you will have reduced power if the mis-fire does not return them I would certainly replace the pick-up coil. Regards, Bob

jimbro · 05-18-2016, 06:02 PM

Quote:

Originally Posted by bobmiller1

Being that the problem is "RARE" but still happens when the tank is full may lead to not a fuel problem but rather an ignition problem...although you tested the pick-up coil...for the price did you just consider replacing it ??? You may want to try driving it with the vacuum disconnected from the vacuum advance...although you will have reduced power if the mis-fire does not return them I would certainly replace the pick-up coil. Regards, Bob

Bob,

Thanks for weighing in. I did remove and plug the vacuum advance line and the sputter still occurred. The issue only happens when the fuel level drops to the lower portion of the tank so that was why I was suspecting fuel especially after replacing the majority of the electrical components. I suppose at this point what's the point of not replacing the pick up coil.
I was planning on installing an electric push fuel pump just ahead of the tank on the weekend but maybe a coil replacement on Thursday can be inserted into my daily plan.
I will keep you posted as to what I find out.

Boom Sounds · 05-21-2016, 02:55 AM

Does it always happen or intermittent? You need to identify if it is fuel supply or ignition. Have you got the EGR set up? If so, check rubber hose which can collapse and valve. Start with the fuel though, it sounds like a fuel supply issue. If you haven't replaced the pump on the rail, usually just forward you should do so. Keep pressure testing until you have the right pressure, I think it is meant to be much higher than you report. I believe there is a filter that can get clogged in the tank itself, but think that is a last resort. Check out chevytalk.org, thirdgen.org and gmtruckclub who are very knowledgeable about these things. Look at Chevytech on chevytalk forums and see if he covers your symptoms. Cody the car guy on YouTube also good, depending on your year model. Do you have obd1 on the coach and is it throwing up any codes?

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Boom Sounds · 05-21-2016, 03:48 AM

73-87chevytrucks forum has some great manuals which show the exploded view for the P serries and Motorhome systems. Look at the electrical wiring diags and it should show all the 12v pumps etc. involved in fuel distribution.

Boom Sounds · 05-21-2016, 05:57 AM

Have a look at the link to the service manuals in my parts thread. You might find some of the fault finding sections useful. I think they specifically cover some of the stuff around missing at higher revs. They are searchable. I think only the earlier manuals, the Motorhome section was within the Light Truck chassis manual, but you would need to check. Good luck!

BTB · 05-21-2016, 08:32 AM

...on a Suburban. Turned out it was a melted catalytic converter. And it would also happen when going up hills.

This is when I learned to change the O2 sensors on a regular basis - 80k miles. The O2 sensors get old, the engine pumps in more gasoline, and the excess gas gets burned in the cat, which over time gets too hot and melts.

I had a hard time diagnosing it too...

Bill

J. Morgan · 05-21-2016, 01:24 PM

Judging by what I have read, I think you have a fuel delivery issue,,, BUT, the mention of a melted converter is a real possibility, and a difficult to diagnose issue that I imagine affects many older vehicles and some newer ones.

The last melted converter I had stymied the "experts", I had to diagnose that one myself.

Superat stultitia.

jimbro · 05-24-2016, 03:10 PM

Well.... after pretty much exhausting the electrical side of the equation I focused my attention to the fuel side near the tank. I tried the electric fuel pump, no correction. Read in a thread somewhere to check the vapor line returning to tank. Thought that made sense so I dropped the tank and low and behold the line is completely squashed coming from the tank! Not sure if that is it or not but when the tank is back in, maybe tomorrow, I will let you know the results.
40 years of pulling back yard wrenches and maybe I got to learn something!
Great day! What a journey!

Isuzusweet · 05-24-2016, 05:09 PM

I know what I'm about to say has been said over and over again, so if I sound like a broken record tell me to bugger off.

While you're mucking about with fuel lines, how about replacing the lot as they are 30 years old and pre Ethanol. Ethanol eventually degrades the vintage fuel lines to a point that they start leaking. We have lost a number of coaches in the past few years due to engine fuel fires and that dinky fire extinguisher inside is just enough to piss the fire off, not put it out.

By the time one owner, on his maiden journey after an extensive restoration, had his Mom phone him to say smoke was coming out from beneath his coach, it was too late.

Cheers
Tony

Boom Sounds · 05-31-2016, 11:54 PM

Tony - what product would you use to replace the fuel lines. I think my stock lines are a combination of rubber and steel.

Cheers
Nick

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J. Morgan · 06-01-2016, 10:26 AM

Use a good brand high quality rubber hose to replace the rubber portions of the current lines.

I would not replace the steel portions unless they have corrosion issues.

Superat stultitia.