Misfire under load

Rambunctious · May 27, 2014, 12:28 PM

Okay...I'm sure this subject has been dealt with here before but I could not find it. I have a 1987 Class A 454 Chevy. It started to misfire when going up a hill I rebuilt the Carb replaced all plugs and plug wires and it still missed badly at the slightest up grade. I found a fuel filter on the passenger side rail and with great luck I found that exact vintage filter on ebay. I put it on and the engine ran perfect, it idled down properly and ran real smooth. It took on the hill just fine until I reached the top and then it misfired a couple of times. (DANG IT) Not as bad as before but it still did it.
When it misfired I noticed the temperature was over 200 and I didn't hear the fan kick on. I'm going to get heat sleeves for the plug wires and maybe go through the cooling system...new t-stat or fan clutch.
Can anyone think of something I have missed? On a scope it is picture perfect. i??

DonD · May 27, 2014, 12:47 PM

That side rail filter is quite common btw. $12 or so at NAPA and others.

Change the carb inlet filter. Take out the old and match to be sure.

cncsparky · May 27, 2014, 12:57 PM

Have you pulled the tank and checked the in tank fuel pump? Mine is a '87 chassis and I found a broken rubber fuel line inside the tank that connects the booster pump. Very common problem to find that could cause a vapor lock issue.

http://www.classicwinnebagos.com/forum/index.php/topic,9139.msg50576.html#msg50576

Rambunctious · May 27, 2014, 01:32 PM

Quote from: DonD on May 27, 2014, 12:47 PM
That side rail filter is quite common btw. $12 or so at NAPA and others.

Change the carb inlet filter. Take out the old and match to be sure.

Thanks I will try that.

Rambunctious · May 27, 2014, 01:35 PM

Can I check the fuel pump an easier way? I thought I could remove the fuel line from the carb and lead it into a jar pull one plug wire and crank the motor. Did I just make sense? lol

DonD · May 27, 2014, 01:51 PM

Ramb, I would pull the inline filter, attach a piece of hose into a container..then turn on the ign.to see if it's pumping.

On mine I can pull the gas cap and listen to it running to determine if the pump is at least running, then do the above test...Mine runs always independent of oil pressure as long as the ign is on.

DaveVA78Chieftain · May 27, 2014, 02:02 PM

QuoteCan I check the fuel pump an easier way? I thought I could remove the fuel line from the carb and lead it into a jar pull one plug wire and crank the motor. Did I just make sense? lol

The plug wire you refer to is the coil wire, not just a plug wire. Still, both the coil and fuel line are on the front side of the engine so there is potential for a spark igniting that jar gas. Best, and safest, way is to temporarily install a cheap fuel pressure gauge at the carb end to ensure fuel pressure is in the 5-7lb range.

Be sure to check operation of the mechanical and vacuum advance in the distributor.

Dave

cncsparky · May 27, 2014, 05:47 PM

Connecting a fuel pressure gauge somewhere in line before the mechanical pump would be best to test the in tank pump. If your line is split like mine and others, you could get fuel out of the fuel line with the pump running but no pressure. The pressure guage would tell you if there is a problem.

Dave, putting a pressure gauge at the carb would not tell him anything about the in tank pump IMO. Once the in tank pump/fuel pressure regulator is verified good, however, the pressure into the carb would be a good check to do.

cncsparky · May 27, 2014, 05:52 PM

How old is the cap/rotor? Anytime I had a misfire under load in other vehicles, it was something to do with ignition. However, vapor lock can be an issue in these RV's so good to go through the fuel system too. Especially since you mentioned it running hot.

What plugs did you put in it?

DaveVA78Chieftain · May 27, 2014, 06:45 PM

QuoteDave, putting a pressure gauge at the carb would not tell him anything about the in tank pump IMO.

.

Whether you start at the carb or at the input to the mechanical fuel pump is moot. You need to check through the system as a whole.

Dave

cncsparky · May 27, 2014, 08:51 PM

Thats fine Dave

. It sounds to me like classic vapor lock problem, so personally I would start where those issues begin, the in tank pump since he should have one. i??

bluebird · May 27, 2014, 09:57 PM

I agree with the cap and rotor. GMs with the HEI like new caps and rotors pretty often. I use the brass terminal caps from NAPA, they last a bit longer than the aluminum term caps.

Neil Carrick · May 27, 2014, 11:00 PM

Just this past fall.. our 454 started to backfire when coming down a hill, and it stalled just short of the yard at home. Turned out the coil burned off one of the wires to it inside the cap.. arcing inside.
New coil, cap and rotor along with plugs and wires.. and good as new.
Neil.. Hm?

legomybago · May 28, 2014, 02:02 PM

Everybody has a differant view on what a misfire is.....like, "does it cough?, does it just fall on it's face? does it just feel like a vibration?....All we know about this situation is 'heat" and "under a load". So like everyone else on here has said....it's either a disributor issue or poor fuel volume causing vapor lock.

tiinytina · May 28, 2014, 06:40 PM

same thing but it was the wiring under the "pick up" unit that is on the rotor cap, drivers side. The clip had failed due to age thus wires were not tight ... not before it backfired so violently that it blew holes in the manifolds and mufflers... zip ties later.. sounding like a tank... made it home 90 miles later.... Banks, Magnaflows and new pipes, the pick up is still ziptied up and doing fine and she sounds like a pack of Harleys. knock on wood.

pvoth1111 · May 28, 2014, 06:43 PM

timing running out of advance? or the vac advance just plain not working

legomybago · May 29, 2014, 11:37 AM

Can't be a timing advance issue, he's getting to the top of a hill before it misses. He would have issues all over the board and a lot sooner if his ignition wasnt advancing correctly. IMO

Maybe today will be the big day he writes back and says "I fixed it"!!! "Here was the problem"

Lets hope

Rambunctious · May 29, 2014, 06:13 PM

Okay...I picked up a new Thermostat. I got a High performance T-stat set at 180. I noticed that my fan wasn't kicking on. May need new fan clutch. I think at the top of a hill when my engine is at it's warmest is when I'm getting this problem. After I replaced the filter on the side rail it greatly improved the performance of the engine and reduced the misfires except when my RV is at or near the top of a hill. The temp gauge was at 210.
I think I had two problems with similar symptoms.
I had replaced the plugs with cool plugs and heavy duty wires and I picked up some heat sleeves to keep pre ignition from occurring.
I have a feeling that if I can keep her running between 180 and 190 degrees it will solve my problem. I hope. I had my shop hook up a scope and they said it's textbook and everything looks fine... i?? So Im leaning towards it being heat related. If that doesn't work then I'll put a fuel pressure gauge on it and see what it reads.
Ahhh the joys of RV ownership...

Rickf1985 · May 29, 2014, 06:57 PM

210 under load is not bad at all. You could be vapor locking though. I put in a new fan clutch for good measure when I put in the water pump and it kicks in too early. as soon as the thermostat opens the fan clutch kicks in for at least a minute. That certainly is Not helping my mileage! Cruising down a striat and level highway at 60 it is in and out, in and out.

pvoth1111 · May 29, 2014, 07:26 PM

Yea....sounds like timing...use a light....

legomybago · May 30, 2014, 10:35 AM

Yeah Rick, we put in the most severe duty fan clutch they make and it would kick on and off just cruising down the highway..PIS*ED me off!! But after putting 4 or 5 hundred miles on it, now it works as it should? Comes on at the top of the hill, then shuts down as you head down the other side...Never kicks on just cruising the highway now. It needed to get broke in i guess

Rambunctious · June 02, 2014, 11:45 PM

That's good to know...

GONMAD · June 03, 2014, 11:01 AM

Hi Guys, This is from prior experience. The HEI has a major design flaw insomuch that the gap between the rotor button & distributor mainshaft is very close & the importance of a good quality cap & rotor with brass terminals is paramount to preventing a "jumping" short to the mainshaft in the cap. You will never SEE this (unless you have a clear aftermarket cap). Make sure you have a good aftermarket hi performance coil as it helps. Electricity travels to the point of LEAST resistance & a good ignition system is required in heavy duty applications. Also you may try a remote coil system as this seems to help in some instances. Just a thought for your preponderance. Hope you have a good day GONMAD

Rambunctious · June 06, 2014, 10:41 PM

Success!!!!! Problem is solved. I replaced the thermostat and the fan clutch. I put heat sleeves on my plug wires and I went the extra mile and put in a larger radiator. I took it out for a drive and we went on a search for the biggest hills we could find and it didn't skip a beat. It never misfired once. We were running at about 180 degrees up and down hills . Outside temp around 90. I tested the fuel pump (OK) and replaced/upgraded the coil as suggested here. So whatever it was we fixed it. Thank you all very much.
We can make that trip now.

DaveVA78Chieftain · June 06, 2014, 10:45 PM

Great!

Always glad to hear someone got the problem fixed. Enjoy your trip.

Dave