Mystery Problem

[Steamer's Wake]

I have a 92 RR with about 60 k miles on a 4.0 replacement engine. It makes a banging backfiring type sound under load at about 2000 rpm. It runs perfectly if you rev the engine w/o load to 5000 rpm. There is some loss of power when you have the noise.

I have had it two mechanics working on it.

So far: new plugs, new wires, new distribute cap. Checked timing, checked compression, removed and tested catalytic converters, replaced coil, replaced wires a second time, new fuel pump, replaced fuel with new fuel, new fuel filter, had the fuel injectors cleaned, fuel pressure is fine, new temp sensor on the injection. Oil change too. Check engine light is not on. Rovercom shows that the engine is running fine with no errors.

Ok what is it?

[TSR53]

I'd make sure that all your grounding wires are up to spec and possibly check the voltage regulator as well. Looks like you have all the other items covered.

[rovertek]

with the loss of power its making me think (plugged catylitic converter) which would not send a code also.....

[Steamer's Wake]

Thanks

Thanks,

One of the mechanics thinks that it could be a plugged cat, so you may have something there. The grounds sound like a possibility too. Thanks for the suggestions.

[Steamer's Wake]

Not the Cat

We switched out the converter for a good one and it did not change anything. We also tried another new set of wires.

[MJL]

Have you checked to see if you have sticking exhaust valves. See the write up at:

http://www.robisonservice.com/servicedep/pdfs/carbon_fouling1.pdf

Mike

[Steamer's Wake]

Thanks for the reply

Thanks for the report. I will pass the info on to my mechanic. Wouldn't a sticking exaust valve show up in a compression test?

[phoenix]

what is the condition of your new plugs, sooth?, burnt?, gap?. is your engine puffing smoke or not?, have you tested the trottle position sensor.
earlier cux14 engine management sytems were archaic at best, engineers
had convinced the market that a set of 02 sensors would fix the emmisions problems of the past, we know now it is marginable true. that having been said, do not rely on sheer codes for a diagnosis, that can get you in trouble by replacing parts you do not need to replace. "K.I.S.S"
follow a basic trouble shooting path,we know the primary and secondary
ignition systems are in good shape, next have you checked alternator output?, if under load you loose power the coil looses current and it backfires as it hunts for current (sort of a rev limiter).
are you starving of air?, check the airflow meter. what about vaccum advance?, basic timing?, crossed ignition wires? inside the distribuitor are the advance ballasts free to move?, do they return to the normal position?
are the springs worn? (ballast springs).
we could assume that with such low mileage the timing chain is still good
but have you made sure?
all of these things need to be dealt on a one by one basis before the problem is solved, any mechanic half way competent should be able to make and follow a basic trouble shooting routine and eliminate the guess work and not just base himself on how much he may net after a repair.
that having been said, sorry for my ranting and the best of luck.
feel free to write if all checks fine and problem is not solved.
it may end up being a plugged fuel filter.

[Steamer's Wake]

The engine does not puff smoke. I don't know what the plugs look like. They only have 300 miles on them though. We tested the throttle position sensor and it is working correctly. The timing is correct and is advancing correctly. We switched out the airflow meter with a known good one. The ignition wires are correct and not switched. On the distributor we switched it out and tried a know good one and got no change. As for the timing chain, when we put the engine in, we added a "special" part to reduce wear. The fuel filter has been changed, and the injectors professional cleaned as well as the fuel rail cleaned. We think that we are on the path to having the valve train cleaned to see if it makes a difference. My mechanic suggested that if it does that if we are going to do a valve job, that we should change the heads from the 4.0 heads to the 3.9 heads.

[Steamer's Wake]

The timing chain is a Double Roller timing chain.

Steamer's Wake

[phoenix]

hook it up to an oscilloscope, so you can see the firing pattern, i suspect it may have something to do with loss of energy/power, see if by any chance the backfire gets worse when the lights are on, also diconect the battery for about a minute so the adaptations reset on the ECU, of course that is done with all off.
if the back fire continues look into the alternator not having enough output to maintain charge, make sure terminals are secure and clean. also there is a body to engine ground at the starter that normally gets forgotten on engine swaps.

[Steamer's Wake]

Backfire through intake under load

Well,

The battery terminals are clean, the grounds checked and all are fine. The alternator is working fine even under load. The compression was checked again and it is fine. The number 3 spark plug is very black worse that the rest. We replaced that plug. The plugs all look like the engine is running rich. We switched out the distributor again, retimed it and nothing changed. We ran water over the valley and around the top of the engine looking for a vacuum leak. We checked the plug wires and all are firing correctly and the coil wire is fine. All the wire and plugs are new. We checked the fuel to see if it was aerated and it did not have any bubbles in it.

So what can it be?

[joshua.mcguoirk]

Crossed wires.

I read that you checked the wires... However this is indicative of a firing order mishap.

[Dan]

Does it back fire when you rev it up under the hood by hand or just when you are driving!

[Dan]

I have had experience with fuel injection systems that have the o2 sensors fail and give the computer wrong inputs obout fuel mixture. When they where doing this the fuel wixture became rich which inturn fouled the plugs. Have you unpluged the o2 sensors to see if it makes a difference! just an idea!