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Old 05-30-2004, 10:03 PM
boothill66 boothill66 is offline
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backfires under load when hot

I have a 1987 dodge ram 250, class b motor home (or just a big old heavy van) with a 318 engine. It is backfiring under load usually only when it is warmed up. Getting up to speed on the highway is almost impossible but once itís up to speed itís fine on level ground. It backfires on inclines but if I downshift and get the RPMs up I can push it a lot without any problems. Any more than about an inch on the peddle causes backfiring only under load and only warmed up. It has new plugs, wires, cap, filters (fuel and air). Took it to Firestone Service but they canít fix cars anymore without a computer printout. But they tell me the timing is ok and whatever sensors they can check are working. I just ran most of the gas out of it and filled it up (35 gallons) and took it out again, the mileage is the same if not a little bit higher.
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Old 05-30-2004, 11:19 PM
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Welcome to the forum.

I would consider looking at your carb and checking all of your vacuum lines. Another thing to check would be your vacuum advance.
Just some ideas.
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Old 05-31-2004, 07:23 AM
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Yeah Beefy, I'm thinking ignition timing also.
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Old 05-31-2004, 12:26 PM
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don't rule out the egr valve.
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Old 05-31-2004, 12:53 PM
boothill66 boothill66 is offline
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The timing was the first thing I checked it was exactly as specified but jumped around a bit. The vacuum was a rock steady 18 and I would think if the carb were the problem it would be rich, thatís why I did the mileage test but I guess I can check it anyway. The last engine I worked on was on my ë67 Goat but this van has just enough computer controls to make it confusing but not enough to make it diagnosable and someone told me that it could be in the emission controls, specifically the air pump. Donít forget, it did spend 2 days in the shop and they checked all the basic stuff after I did. This is actually a big doghouse and my mutt is becoming very depressed the doghouse
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Old 05-31-2004, 07:23 PM
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I had worked on a 1990 dodge truck about a year ago that had worn the distributor drive gear and distributor shaft bushing (in the block) down to almost nothing. I found out from the Dodge dealer when ordering parts that this was a very common problem with them.

Anyways, since your in the dog house (pun intended), pull out the distributor and look at the drive gear for exessive wear, also look in the distributor hole and visually inspect the bushing in the block for wear.

If it is noticabley worn, you will need to replace the cam & cam bearings, lifters, distributor drive gear, distributor bushing in distributor, distributor bushing in the block, timming chain and gears and all neccessary gaskets.

The truck I worked on was a V6 engine if I remember correctly, but it did the same thing as your van and when I tried checking the timming, it was jumping around like yours......because of the worn gears, chain and bushings.

The distributor drive gear and bushing in the engine block showed the most signs of wear due to the soft brass that they were made from.

I hope it was just the V6 that had this problem and not your 318, but I just don't know. It's not a quick and easy fix, not cheap either.

Good luck, and keep us posted!
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Old 06-01-2004, 01:09 AM
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I'm with the cat. MOPAR's are notorious for weak timing chains. How many miles on the family truckster?
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Old 06-01-2004, 12:39 PM
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The V6 and V8 318. If you look at them the V6 looks just like a chopped 318 block.
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Old 06-01-2004, 06:38 PM
boothill66 boothill66 is offline
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97,000 miles and 7000 miles was done in 10 days last year crossing the country including going over the Rockies without any problems at all. I put about 1000 since Iíve been back and the thing to remember is it started suddenly not like something was going through normal wear and tear. But I would think the chain or distributor gears would be just as much of a problem when cold. So Iíll start with the carb then the EGR valve because I can do those from the captainís chair with the overhead AC on and AC/DC in the DVD player. Itís sure the easiest place to start wouldnít be a waste of time on an engine this old.
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Old 06-02-2004, 06:46 PM
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Don't forget, the oil is thicker when cold therfore it might keep the parts a little tighter until it warms up.

I say it is the distributor or timing chain. 97000 is not very high milage anymore therefore it does not sound like it is the chain. As the reader above mention that the distributoir gear worn out because of cheap quality unless you are real lacking in changing the oil.

Backfire is caused by having unburned fuel in the exhaust or intake manifold and that can only happen when a spark comes by or hot exhaust from an open valve on the exhaust stroke.

A spark at the wrong time is most likley the cause, therefore timing, etc.

Also, on some cars there is one ignition coil per two cyclinders and both plugs will fire at the same time. That means as one is in the compression/power stroke, the other cyclinder is in the exhaust to intack stroke and the valves are open. That is one way you get the backfire. Valves are open and there is spark because the distributior is way off and the spark did not come by when the vavle were closed.

I assumed you have checked the distributor cap for cracks, moisture, etc.

That real slow build up of speed is from a retarded spark and it must be firing too soon therefore you are not fully in the compression mode.

On new cars that have a knock sensor, when the car pings too much the sensor detects the noise and the voltage to the computer changes and the spark is electronical retarded. This was a common problem on the very first year the Saturn SL1 came out (1991). Until one engine mount was redesigned the engine vibration was fooling the knock sensor and the sympthom was a sudden lost of power.

A cold engine the fuel is much richer and other parts are colder therefore it may take more to cause a backfire.

Don't waist time on the carb.


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Last edited by gsica; 06-02-2004 at 07:02 PM.
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Old 06-02-2004, 08:55 PM
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Jeez.......I had the same problem this winter.......it was the carb......dont waste time with it though.......it couldn't possibly be that.........

Suspect everything, all of these posts were good posts until gsica said don't bother with the carb. Go through the list, check everything, and don't replace anything until you KNOW a part is bad.

Unless you are like me and have a boat load of parts laying in the garage.
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Old 06-02-2004, 08:57 PM
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2 things your post doesn't tell us and I think many of us are assuming.
2 or 4 barrel
backfire from carb or from exhaust?
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