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1975 CJ5 304 V8 Sputtering and backfiring

4311 Views 38 Replies 12 Participants Last post by  Jschenck1967
Good afternoon, this is my first post, and question. I have recently been rejuvenating my Jeep, and as part of that i have needed to replace a few parts. Be it noted, i am not a Mechanic, so play nicely. My sons (17) and I as part of the quarantine have been working together on this thing, as project based learning. We have replaced the gas tank, which had rusted out, blew out the fuel lines as part of that. Replaced the headers, which had rusted out. The new headers are nearly identical, however the new mufflers are chambered, more noise, less pressure (I am guessing) We have also done some remedial suspension work. (shackle bushings, shocks) we have also replaced the tires, brake shoes and some other minor cosmetic items. we also did basic maintenance, oil change, add antifreeze...We are looking to rebuild the carburetor soon. My attitude has been if it ain't broke...don't fix it.

This Jeep, up to yesterday has started and ran nearly perfectly (with the exception of an electronic ignition issue in the 80s) it is now rough on the idle, and inconsistent starting on the first turn.

The issue came up yesterday when the boys took the Jeep for a trip to the Gym, then the market, back roads about 5 miles there, the car sat for 1 hour, then they ran a quick errand to the market. Jeep sat 15 minutes. On their way back home, they took the highway, traveling around 55 mph and about half way home, the jeep began to sputter, back fire etc. They called me and I ran out to help them, thinking i would tow them home. Turns out i started the Jeep, it sputtered, and popped (no backfire or flames out the pipes) however it struggled to get to a higher rpm. I got the car up to speed by putting it from first to third (3 speed) then i was able to get it home. Once home I immediately parked in the garage to let it cool. I noticed two things; there was a large bulge/bulb in the top radiator hose where it enters the engine. second there was a slight radiator leak out the overflow, maybe a teaspoon. I have driven it since and could not recreate the popping or backfire, though i did not drive if for the duration that the boys did.

I have found other similar posts and threads which i tried to paste the links but being a newby i was not allowed.

check:

vacuum
carburetion
fuel line
points
etc.

I will follow the suggestions in those threads but would also like a bit of advice, if you see something here that catches your eye.

Thanks in advance
JS
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You've not been able to recreate the event? Does it still lack power?

How hot was it?

Is the fuel filter oriented correctly?

How much pressure are you building up in the cooling system? Was that hose new, and bulged, or was it old? If you've got a head gasket blown, it could possibly put too much pressure in the cooling system. But you also note no smoke/steam out the tailpipe, so I'm hesitant to suggest this.

Josh
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hello
pull the spark plugs and read the best sensor for combustion. they should all be a light black gray color. if one is alot different then you are having a combustion problem in that cylinder.(prob a intake leak). if there all the same you prob have a fuel problem.

since you change out the fuel tank because of rust there are things that need done. i did see you blew out the fuel lines. hopefully disconnected everything first. then from the tank connection to the fuel pump , then line from pump to filter, then filter to carb. did you change the fuel filter? one thing to check is the fuel pump pumping enough fuel. disconnect fuel lin from carb and run rubber hose to a mason jar. turn over the engine for 30 seconds. you should get about a pint. if that or more the pump is good. if less try without the fuel filter. if good with no filter attached then the filter is bad. if still not a pint then the pump is bad.

depending on how long that jeep sat. the carb could need rebuilding. where something like varnish or junk is making something stick.

oldschool
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Josh thanks for the response,

I have not been able to recreate the event, though i have not driven it to the point where it was hot. However, there seemed to be normal power for the two mile round trip i took.

Yes, the fuel filter is oriented correctly, that was replaced with the gas tank, the tubing sizing does not allow it to be mis-oriented. Also i have driven it about 100 miles since that was installed.

I do not know exactly how hot it was...how would one check that, without an infrared thermometer?

The hose i believe is original equipment (45 years)...yes I'm gonna replace that as soon as i can.

I too was concerned about the head gasket, given the bulged hose. But as i have read, there would be other symptoms, milky engine oil, white smoke out the exhaust. I don't see any of that.

with only 68k on the engine, i am hesitant to think it is a timing chain- I am reading up on how to check slack without disassembling...

Old School, I will look at the spark plugs

And i have a rebuild kit for the Carb sitting on the shelf, will get to that soon.

when we replaced the tank, we did push air back to the tank, from before the fuel pump, and yes the filter was replace (correctly), and the fuel pump is pumping, I replaced that a while back, and it did put pressure into the lines, though i did not crank for 30 seconds as you suggest. i will do that


John
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Josh thanks for the response,

I have not been able to recreate the event, though i have not driven it to the point where it was hot. However, there seemed to be normal power for the two mile round trip i took.

Yes, the fuel filter is oriented correctly, that was replaced with the gas tank, the tubing sizing does not allow it to be mis-oriented. Also i have driven it about 100 miles since that was installed.

I do not know exactly how hot it was...how would one check that, without an infrared thermometer?
I had a thing for aftermarket gauges in my '76 CJ5. The engine was warmed over, so that was part of the reason, but mostly I wanted to know what my engine was doing. Just installed a vacuum gauge on my 2004 Grand Cherokee, in fact. I consider tachometer, vacuum gauge, oil gauge, and water temp gauge essential, especially on older, pre-computer engines.

I've used a food temp thermometer in the overflow hose in the past. Not really recommended due to scalding danger and possible imprecise reading, but it worked for that time and place.

The hose i believe is original equipment (45 years)...yes I'm gonna replace that as soon as i can.

I too was concerned about the head gasket, given the bulged hose. But as i have read, there would be other symptoms, milky engine oil, white smoke out the exhaust. I don't see any of that.
OK, old hose. No problem.

with only 68k on the engine, i am hesitant to think it is a timing chain- I am reading up on how to check slack without disassembling...
If it were the timing chain, you'd notice it other then when hot.

I think it's heat making something break down. Either a bit of vapor lock, or could be a bad ignition module. Or, like Oldschool said, the carb could be sticking.

Josh
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Possible vapor lock or carburetor heat soak?

" the boys took the Jeep for a trip to the Gym, then the market, back roads about 5 miles there, the car sat for 1 hour,
then they ran a quick errand to the market. Jeep sat 15 minutes. On their way back home, they took the highway,
traveling around 55 mph and about half way home, the jeep began to sputter, back fire etc. "

Also - I read in your profile you plan to install wheel spacers for a wider stance. I'm not a fan of wheel spacers.

For model year 1975 the CJ5 wheel option was the 15X7 Forged Aluminum Styled Wheel. Looks like your Jeep
has these wheels in you profile pic. You should be able to run a 31 X 10.50 tire on a 7 inch wide wheel - although
at 7 inches wide that is at the lower end of the approved wheel width range.

I run 15X8 white spoke steel wheels on my '79 CJ5 with 31 X 10.50 tires at about 28 psi and it handles fine.

( The CJ is no sports car! )
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does it still have the prestolight ignition system on it? or did they still run points in 75 on the v8
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hello

the prestolite was started in 1975-77. i just looked it up the other night on amcdepot. someone else that put a 74 304 was having ignition problems.
i forgot about some of the issues the prestolight's had. wasnt really thinking about that because most have been replaced with the motorcrafts. that motorcrafts 99 percent of the time just quite running.

oldschool
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My money is on either ignition or fuel. Have you checked vacuum to the distributor?
I'm betting it's either air, fuel, or spark. >:)

Checking the plugs is always a good place to start.

Matt
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Hi not being able to recreate the problem with short drives my bet is on over heating.. kids well be kids.. bulging hose dripping out the over flow over heating problem... did you have to add coolant? or checked it? have you replace the pressure cap? if the pressure cap does not hold the proper pressure it well cause the loss of coolant and you wont see where it is coming from as it well be loss as you drive.
good luck
tim
My thought is that ignition issues may be causing the overheating issue - A bulged hose is just that, however it still transfers coolant. The OP did indicate that coolant was added prior to the event. Now unless the motor didn't have any coolant when it was acting up, the issue is probably elsewhere. Definitely take care of the cooling system to rule that out (thermostat too). It does sound like a heating issue caused lack of power, but the cooing system would have been pretty dry and the temps high for this to happen.
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I will go through the laundry list of items other folks have suggested. (even though i dont leave the house, i havent had time to sequester myself in the garage...)

Thanks everyone, I will update as soon as I have a chance and have made progress...
I did add coolant, however, will get a new T-stat as well, it has been a while since that was replaced.
My money is on either ignition or fuel. Have you checked vacuum to the distributor?
the Vacuum hoses seem to be a bit suspect, yet another possible culprit
Things never correct themselves, except some times. After you have a look at the plugs drive it up to temperature again and see if it will duplicate the same situation.
I know nothing about motorcraft but know the problems I was having with the Perstolight ignition in the mid 1970s.
Sounds alike.
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HI If you had to add coolant replace the cap ... if it don't hold pressure (as I have said) it well loose coolant and cause overheating .. over heating will cause it to not run right .. you can get a mechanical gauge to replace a stock one and then you'll know what your temp is ..
replace the cap its cheap and can save a lot of headaches from other problems overheating causes..
good luck
tim
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sounds like fuel starvation. Its not a cooling problem, although those weak points ned to be fixed. Is it still getting a consistently strong spark out of the coil? You should be able to pull the center wire from the distributor and hold it about 1/4 from the block and see a consistent spark. WARNING! don't do this by holding it with your hand. Use a well insulated pliers, or a plastic fuse puller, to hold the wire.
If spark is ok, then start looking for lack of fuel pressure. the diaphragms of fuel pumps die over time if not run every 2 or 3 weeks to keep them flexible.
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sounds like fuel starvation. Its not a cooling problem, .
Loss of coolant , causing overheating, causing fuel starvation from vapor locking, also causes ignition not to fire properly.

good luck
tim
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I have finally had five minutes to peek under the hood. I checked the spark plugs, they are all a consistent light gray, which by understanding what Oldschool noted, it could likely be fuel. i have other parts. T-stat, vacuum tubing and radiator cap arriving tomorrow and Friday, perhaps by Saturday I can rebuild the carb, test fuel pump etc. more info to follow.

Cheers,

John

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