1FZ-FE misfire diagnosis. (1 Viewer)

[Tezzadactyl]

Hi all, I have a '97 80 series with a 1FZ-FE engine. It has been modified to run duel fuel. (LPG-Petrol) It starts and runs fine except for a misfire that appears at around 2000 to 2400 RPM. It usually happens when its just cruising and not really under much load. If I accelerate hard, it revs out fine with the misfire gone.
I'm feeling that it could be either the spark plug leads, the spark plugs themselves or the injectors. Any advice on any tests I can perform to try and diagnose this misfire, so I'm not just throwing cash at all new parts, would be greatly appreciated.

Cheers

 

[robnicko]

is the misfire on both petrol & LPG or just on one fuel?

Is it injected GAS or the vapour type (impco type unit on the intake hose between air filter & throttle body)?

What plugs are you running? for LPG a colder plug, NGKBKR6EYA for example is recommended as opposed to a '5'

LPG is very sensitive to faulty ignition leads. You can check each lead to ensure theyre within acceptable resistance specs

ALso, check for vacuum leaks as a small leak can cause hunting / missing under cruise but is hidden on WOT

 

[fjbj40]

Look at the date on the ignition leads, if they are OEM they will have production date.

If they are original it may be time for replacement.

But go cheap to expensive, plugs, rotor, cap and wires.

 

[gummycarbs]

Misfire like popping and sputtering, or misfire like it's just stumbling a bit? Is it literally RPM dependent, or load dependent?

 

[Tezzadactyl]

is the misfire on both petrol & LPG or just on one fuel?

Is it injected GAS or the vapour type (impco type unit on the intake hose between air filter & throttle body)?

What plugs are you running? for LPG a colder plug, NGKBKR6EYA for example is recommended as opposed to a '5'

LPG is very sensitive to faulty ignition leads. You can check each lead to ensure theyre within acceptable resistance specs

ALso, check for vacuum leaks as a small leak can cause hunting / missing under cruise but is hidden on WOT

Thanks for the reply, the misfire happens on both LPG and Petrol. Maybe a little worse on LPG.
It has a vapour type LPG conversion. I haven’t checked what plugs are currently in it but I will do that when I get a spare minute.
Do you happen to know what the resistance specs are for the leads?
An air leak might be possible as the connection between the LPG unit and the throttle body is a bit agricultural!

 

[Tezzadactyl]

Misfire like popping and sputtering, or misfire like it's just stumbling a bit? Is it literally RPM dependent, or load dependent?

Hi gummycarbs, it just seems to stumble a bit and definitely seems to be RPM dependent. As soon as I kick it back from overdrive and the RPMs increase, the stumble goes away.

 

[robnicko]

Thanks for the reply, the misfire happens on both LPG and Petrol. Maybe a little worse on LPG.
It has a vapour type LPG conversion. I haven’t checked what plugs are currently in it but I will do that when I get a spare minute.
Do you happen to know what the resistance specs are for the leads?
An air leak might be possible as the connection between the LPG unit and the throttle body is a bit agricultural!

Maximum resistance: 25 kΩ per lead

 

[Tezzadactyl]

Maximum resistance: 25 kΩ per lead

Thanks robnicko, I'll check them out as a first step.

 

[Tezzadactyl]

Thanks for everyone's input on my problem. I decided to change out both the leads and the plugs and it appears to have fixed the problem.