3.9 efi running rough and backfiring

Evening guys

Dad's got a 90 with a 3.9efi in it, always run really well until about a month ago. I ran it very low down in the fuel tank then filled up and drove the 3 miles home without a problem.

The next time he used it it has started to run rough and backfires, it usually does it about half way through the revs but can do it higher or lower as well. its also running rough

I changed the fuel filter and took it for a run but it's still the same, any ideas?

Thanks in advance, Harry

what are your plugs like?

plugs, leads, air leak, endless list. Either get some data on lambda values or emissions - gas analysis, or when were the service items - plugs, leads, coils, distributor cap/rotor done?

Ok raving man seems you guess and are clutching at straws please can you ask what the spec is
Not all 3.9 have lambdas
Do you know what type of leads to recommend?


To the crocks of the problem
Have the injectors tested or cleaned is the pump pressure ok
Suspect fuel pump one poss injector blockage two

Thanks for the help advice guys. I checked the plugs and they are dry/black which i think is carbon fouling due to overfueling?? -correct me if i'm wrong please

took off the dizzy cap (which is only a year old) and there are obvious signs of arcing. I've got a new set of plugs which i'll fit today and put a new cap/rotor arm in to see if that cures it.

Strangely the problem is getting a lot better, it doesn't backfire under normal driving at all now, just under full acceleration and quite high up the revs?

so where do I go from here?

Thanks, Harry

Ok only fit gen dealer ignition parts only

Here's some useful information, which you asked for.

Carbon on plugs only indicates incomplete combustion. It can be due to several reasons including blocked air filter, rich mixture - too much fuel, ignition fault.

In general terms of fault rectifying, you start with all ignition components, effectively you renew all of them, plugs, leads, distributor cap, rotor arm (if it isn't a distributorless ignition system). The reason for starting here is that they are cheap parts and you can fit them easily.

For most problems, just doing the ignition stuff usually sorts out a large percentage of misfires.

But then you get onto the more challenging stuff.

Carbon on plugs can indicate the wrong heat range, but if you are using standard engine and fuels, the standard plugs are good enough.

Some manufacturers say that their ignition leads fit certain vehicles, however, Bosch leads on some vehicles have proven to have poor insulation, which breaks down quickly with use. Magnecor leads are a good replacement.

If the manufacturer uses carbon fibre leads, don't go to low resistance copper leads, because you could burn your coils out vey quickly because of the current being higher than normal.

As Vougese39 says, standard ignition components for most car manufacturers are good and there is no real justification to change. Most marketed ignition changes are hype.

Make sure that the plug gaps are correct, because if they are too large or too small, you end up with poor/variable ignition, which acts like a misfire. I had one garage tell me that one cylinder was down on compression, with a minor misfire, when I checked the plugs, one had closed up - I think the mechanic had dropped the plug and closed the gap up. Reset the gap - perfect.

Ignition problems usually show up with higher load/throttle or engine speed, because the mixture (higher charge density) is harder to ignite.

Sometimes the symptoms you describe can be attributed to a poor batch of fuel.

You might need to run a dose of fuel system cleaner through as a first approach - a bit cheaper than getting the injectors off and getting them cleaned.

You would need to run a couple of tanks worth of fuel from different places to confirm this. But whilst this is unlikely, it has happened.

The thing with any problem solving is that at first you rule out nothing and then you slowly check through the condition of the engine and every component on it.

We could start with - oh! it's a valve starting to go to frighten you, but we might as well check on the basics first and slowly build up a picture of how well the thing is maintained, and when the different components were changed.

We could look at the AFM later.

You could have a water ingress problem, but if you haven't had rain / driven through a ford, i.e. had dry weather then this isn't likely.

But from your picture - was that what you were doing when it started?

The distributor could be loose - yes it does happen.

On the fuel side of things, how many tanks have you gone through since the problem started?

This is important because contaminated fuel could show up a problem after a fill, but then slowly go over several tanks as the contaminant gets diluted out.

It might be that the garage had a tanker put diesel in the petrol tank, and they decided to just fill with petrol to dilute it out.

At garages, tankers putting petrol into diesel isn't a problem if the ratio is less than 10%, but 10% diesel in petrol is horrible.

Me guessing and clutching at straws?

The bloke said - any ideas

And a further note on the fuel issue, some problems can show up fairly quickly, like with silicon in the fuel - knackers the lambda sensors and is not reversible, but as V39 says about different engine specs, no lambda sensor - no problem.

Thanks for all of the help so far guys, and don't worry Raving Man, that is my 300tdi in the picture . Dad doesn't do any extreme stuff in his 90 and it certainly never goes through water!

My 90 had been without a clutch for the last 10 days so i've been driving dad's v8, the backfiring problem seemed to totally dissapear...until yesterday

We were towing my 90 on a trailer behind his v8 and it drove 50 miles without a single problem. We stopped in a petrol station and filled it up and as soon as we pulled out onto the road the coughing started again.

It gradually got a lot worse with the coughing becoming far more frequent and the engine started to lose power very badly, we stopped for 20 mins then carried on, the engine was still coughing and then after a couple of miles lost all power again. we stopped again for 20 mins and the same thing happened.

In the end we had a friend come out and tow my 90 on the trailer while dad followed in the v8, it seemed to regain almost all power (had been sat in the layby for a good 45 mins) but was still coughing and spluttering. Interestingly this now seems to be down in the low revs whereas when it first cured itself 10 days ago, the coughing was only high in the revs with your foot flat down

I don't know much about v8s at all but where should I start??

Thanks for all the help so far, Harry

With problem solving you need to try and determine when things happen, and if there is any consistency.

At the moment, you seem to have a problem that occurs immediately after filling up with petrol.

On fuel pumps, some are designed to be fully immersed in the fuel, including an electrical connection, because it is only 12V - no risk of explosion, but sometimes the wires or the connector can fail, where the fuel actually acts to provide an electrical short.

With your symptoms, if you have a fully immersed fuel pump, this is the only failure mode that would be consistent.

If not the connector, it might be the fuel pump or less likely, the fuel guage sender.

To check this out you could try filling just a gallon or two at a time, and see if you can run it without the problem recurring, then slowly see how much you have to add before the problem occurs - filling tank to 1/4, 1/3, 1/2, etc over several fills, and running through to half-way down the red, before trying the next higher fill level.

Alternatively, if you could rig up a fuel pressure guage that you can see when you are driving, see if the fuel pressure is dropping randomly.

Another possibility might be to find the wires for the fuel pump and wire an ammeter in series, again so you can see it when driving.

If you can see what is happening to the fuel pressure that would be the most helpful.

This suggestion is consistent with it happening when driving, because the fuel gets shaken up and slops around in the tank, and could be giving an intermittent short problem.

It might be a bad wire in the tank, getting pushed around by the heavy fuel, making and breaking the electrical connection.

With immersed fuel pumps, many are installed vertically, as they are axial pumps, and so the wiring starts a good 5 inches from the bottom of the tank.

If this isn't how the pump is installed, let me know and I'll have another think about it.

If you did the standard fuel pressure testing when stationary, it might show up as normal and not a problem, but it might be the wrong result because that test would not be under the same conditions as when the problem occurs i.e. when driving.

With some cars, with emissions control EVAP systems, you have to be careful how much petrol you put in, because you can knacker the charcoal cannister with getting liquid fuel into the pipe and system that was designed to only deal with vapour.

I don't think this is your problem, but it is something that happens with some cars and trying to brim the tank as high as possible.

Thanks raving man! it does have a td5 fuel tank in it if thats of any help? I took the plugs out and wirebrushed them, the little spring pin in the dizzy was very worn so put a new dizzy cap and rotor arm on and its certainly running a lot better, my mate also had a play with the timing.

It was still coughing in high revs and under load (when towing) so i had a local garage have a look at it, they said the little weights in the dizzy have siezed. I havn't got time to fix it as i need to get my clutch done in my 300 so got it booked in on weds for the dizzy to be rebuilt and timing done properly so fingers crossed

Haven't seen a silicone issue for a few years though think it was a issue on a customers car last year and that was shell fuel

Ok mag core leads are still a very over priced leads kit and you should in the real world replace the cap leads and rotor every two years so for the cost of the magnecore leads you can get the whole ignition kit from Lr gen parts for a few quid more so not worth the extra outlay is it!
As I have already offered get the ecu fault read guessing is more costly and will get you know where the 14 cud system is crude but can and does give good longevity
fault will be stored in the memory and will help to trace and cure the fault

I don't know the specific trigger system in the dizzy, but my dad actually had a fault with the earth wire connected to the base plate (points & condenser trigger system) cause a misfire, back in the 70s.

How well did you rinse the tanks out before you put petrol in?

If you didn't it shouldn't be much worse than burning a bit of oil for a couple of tanks.
Depends how much diesel was left in.

You should try what V39 says, it does save time with some faults.

Raving Man said:

In general terms of fault rectifying, you start with all ignition components, effectively you renew all of them, plugs, leads, distributor cap, rotor arm (if it isn't a distributorless ignition system). The reason for starting here is that they are cheap parts and you can fit them easily.

For most problems, just doing the ignition stuff usually sorts out a large percentage of misfires.

Click to expand...

Sometimes that works BUT first you need to know if your misfire is ignition or injection. If you can't get it plugged in then it is useful to run diagnostic checks with a multimeter which can be used to check all the compnents of the injection and ignition system (with the exception of the ecu itself). Don't forget the 3.9 can misbehave if the battery and alternator are not 100% I have certainly had engine misfires due to poor alternator output.