A

AdamHarley

Member
morning all, i’ve been having rough running issues with my 91 3.9 for a while now. They started intermittent but then developed to permanent misfire. Ive changed leads, plugs, distributor, coil. Engine runs smoother but no power under load. Next thing to try is MAF followed by ECU.

My current MAF is a 3AM Lucas serial number 73242A. When searching the net this brings up a lot of Jaguars references. Most replacements don’t list this serial number as one that matches the new part.

Can anyone help with what will work? Am i ok to just get one for year of manfucature?

Im trying either find a second hand one of borrow one to eliminate/confirm.

Thanks
 
3AM MAF can be checked/tested using a digital volt meter. Do this first.
The later 5AM air flow meter should be alright to use.
I can confirm that it is possible for the ECU to have a fault that means it doesn't read the air flow meter in which case you will need a replacement ECU. Also, is your EFI warning light coming on? If so have you re-set the ECU to clear the fault code(s)? Are you using lots more fuel - is it running rich?
The misfire you describe - does this happen when the engine is hot or cold? is it more noticeable when you are running big electrical consumers eg heater fan on full, lights on and heated rear screen on? If so, check output of alternator and condition of battery. Poor charging and a weak battery can cause misfires.
 
Thanks for the responses. I have no warning lights at all and the misfire, when it was intermittent was at all operating temps and no obvious links to electrical load.

It does seem to run rich and seems to overfuel when cold as it stinks! The ECU is the next thing to try after MAF

I’ll google how to test the MAF. I’m trying to borrow a known good MAF and ECU from a 93 car if i can before spending yet more money without a fix.

Cheers and sorry for the delayed response....yet another christmas family thing!
 
i had this with my 1990 3.9.

went through everything, plugs leads dizzy cap.. swapped MAF's ECU' O2 sensors, fuel and temp senders.. it would start and tickover fine, got to 1800-200 RPM then start misfiring.

Turned out to be the distributor, there was play on the rotor arm so when the revs increased, the arm was moving and throwing the ignition out.. it was running very very rich.

There is also the ignition amp module on the side of the dizzy. i had remotely relocated it with the lucas kit. unlikely to be that as when it fails the module wont send out the signal.

1. make sure the timing is a 6 deg BTDC.
2. pull the dizzy cap off and see if there is rotational play in the dizzy.

you say you replaced the dizzy was it new ? did you static time it then dynamic time it ?
 
ukadamwest said:
i had this with my 1990 3.9.

went through everything, plugs leads dizzy cap.. swapped MAF's ECU' O2 sensors, fuel and temp senders.. it would start and tickover fine, got to 1800-200 RPM then start misfiring.

Turned out to be the distributor, there was play on the rotor arm so when the revs increased, the arm was moving and throwing the ignition out.. it was running very very rich.

There is also the ignition amp module on the side of the dizzy. i had remotely relocated it with the lucas kit. unlikely to be that as when it fails the module wont send out the signal.

1. make sure the timing is a 6 deg BTDC.
2. pull the dizzy cap off and see if there is rotational play in the dizzy.

you say you replaced the dizzy was it new ? did you static time it then dynamic time it ?
Click to expand...

Hi, yes I i replaced the whole lot, then had it timed. MAF/ECU seems to be the next step...
 
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