View Full Version : Mystery Problem

Steamer's Wake
10-17-2007, 11:26 PM
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? :mad:

10-18-2007, 08:51 AM
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.

10-18-2007, 10:05 AM
with the loss of power its making me think (plugged catylitic converter) which would not send a code also.....

Steamer's Wake
10-18-2007, 11:25 PM

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
10-29-2007, 10:24 PM
:mad: We switched out the converter for a good one and it did not change anything.:mad: We also tried another new set of wires.

10-30-2007, 11:46 AM
Have you checked to see if you have sticking exhaust valves. See the write up at:



Steamer's Wake
11-05-2007, 01:25 AM
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?

11-05-2007, 06:46 PM
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.
:D :D :D

Steamer's Wake
11-07-2007, 10:46 PM
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
11-08-2007, 09:52 AM
The timing chain is a Double Roller timing chain.

Steamer's Wake

11-08-2007, 05:58 PM
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
11-12-2007, 10:37 PM

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?

11-14-2007, 06:39 PM
I read that you checked the wires... However this is indicative of a firing order mishap.

11-25-2007, 09:19 AM
Does it back fire when you rev it up under the hood by hand or just when you are driving!

11-25-2007, 09:37 AM
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!

Steamer's Wake
11-28-2007, 12:19 AM
Does it back fire when you rev it up under the hood by hand or just when you are driving!

I thought it just did it under load, but it will do it when you rev it up by hand under the hood.

Steamer's Wake
11-28-2007, 12:24 AM
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!

We did try disconnecting the o2 sensors with no change.
We also checked everything for vacume leaks.
I think that it backfires less when the engine is cold.
I found that if i get it to kick down into passing gear, the backfiring was not there. So at low rpm light load everyting is fine. About 2200 rpm under load, the problem starts. At high rpm under load no problem.

Steamer's Wake
12-01-2007, 12:59 AM
We replaced all the spark plugs, taking out the ones that have all of 750 miles on them. We also checked the wires for proper resistance. We changed the temperature sensor for the ecu. No Change. We tried another known good coil. We are running it on a known good ecu. We changed out the distributor cap, with two new ones to make sure we did not have carbon tracking.

I have found that if I push it hard and it kicks down into passing gear the intake backfire is gone until it shifts back up.

12-20-2007, 09:02 PM
I've had similar probs with my 91 rrc caused by corroded wiring around fuel injectors and temp sensors. Strangely it caused weird thumping noises and loss of power but rarely trip a code.

Steamer's Wake
12-22-2007, 01:16 AM
We found a flat cam. The lobe for number 3 exhaust was almost completely gone. The cam was from Rover's North and was Land Rover Cam. We replaced it with a crane cam that ground for a bit more performance. It runs great now.:thumb-up:

12-22-2007, 08:44 AM
Wow! Hard to amagine a hole cam lobe worn down but shure enough that would do it. I am glad that you figured it out! I know how it feels when you finnally get to the bottom of something that you have worked at for so long without getting anywhere!

12-23-2007, 03:32 PM
glad to read you got it, remember my sugestion of using an oscilloscope? that would have given you an orientation of a bad valve or cillinder not firing correctly not nescesarily a misfire.

Steamer's Wake
12-23-2007, 05:07 PM
Thanks for all the tips. The oscilloscope
was a good tip I just didn’t have access to one. I kept looking for one that we could use. I agree that it would have been great to use one early. I am also glad to have solved the problem. The performance cam we put in is great no noticeable idle issues and it seems to rev and run better. We live where there are a lot of mountain passes and I like the cam. Now on to my blown transmission cooler line….