mystery mid rpm misfire

Yeh i know, yet another misfire thread

Any help much appriciated before i lose all my greying hair!!
despite being an lse some clever person (not so) dropped a 3.9 pre serp engine in old rusty before i bought it.
I have lpg and the running problems appear slightly different on each fuel
on petrol it starts and drives perfect when cold, maybe a little down on power but no misfire, once warm the idle occasionally drops right down to feck all but doesn't quite stall. it generally drives fine below 2000rpm then goes loopy up to 2800 above this it again revs cleanly, it won't even cruise without going spastic between above revs and to get it to accelerate i have to kick the old girl right down to get the revs up.

on lpg the symptoms are very similar except it idles worse and does stall now and then but the mid rpm misfire doesn't seem quite as abrupt.

it's got a clean fuel tank, new fuel lines and filter. new leads,plugs etc
i did find the main ht lead was shorting to the - on the coil so gave it a good clean and solved that but i'm at a complete loss as to what's causing it

There are so many things that could cause the problem you describe. In the first place, do you think this is an injection misfire or an ignition misfire?

I would suggest first of all do the basic checks:

Ignition timing, vac advance and mechanical advance.
Battery voltage, alternator output.
Check all vac lines for splits/leaks.
If none of that helps, I might be tempted to pop the plenum off and clean it and the ram housing, throttle disc and throttle mechanism. Don't for get to check the setting of the throttle disc whille you're in there (not likely to cause the problem but while you're at it and all that).

Next I think I would be looking at the air flow meter, check CO Trim.
Then I'm afraid you are in the needle in a haystack position checking sensors - Coolant Thermistor, Fuel Temp. Thermistor, Throttle Potentiometer. If all those check out, you'll need to try a substitute ECU.

Hope that's useful.

maf sensor?

Thanks for the speedy replies fellas,
i'm assuming it's an ignition problem as it still plays up on gas?
but i'll be honest it's been so reliable til now that most of what you mentioned went straight over my head kev
I'm going to put up with it today and go play in the snow, i'll have a look and scratch me head over the haynes tomorrow

i did try and have a look at the vac advance a while ago when it started playing up, does the vac move the baseplate? i tried sucking on it and it didn't feel good i also tried blowing the other way to the plenium to see it it was blocked and it was is that normal? i assumed the stepper motor shuts it off?

rustyrangie said:

Thanks for the speedy replies fellas,
i'm assuming it's an ignition problem as it still plays up on gas?
but i'll be honest it's been so reliable til now that most of what you mentioned went straight over my head kev
I'm going to put up with it today and go play in the snow, i'll have a look and scratch me head over the haynes tomorrow

i did try and have a look at the vac advance a while ago when it started playing up, does the vac move the baseplate? i tried sucking on it and it didn't feel good i also tried blowing the other way to the plenium to see it it was blocked and it was is that normal? i assumed the stepper motor shuts it off?

Click to expand...

Yes the vac advance does move the base plate. If you can suck air back through the pipe vac advance probably shot. You say blowing through the pipe suggested it was blocked?? That doesn't sound right. IF you can find a vac unit I'd be tempted to get one even if you keep it as a spare.

Regarding the stepper motor, it does not shut off the vac advance; all it does is allow the ECU to adjust the idle speed when not moving and no throttle applied.

Thanks again kev, just popped home for a coffee
i tried sucking the vac pipe to get the base plate to move, i'm just sucking in air so i guess the bellows are shot??
when i tried blowing/sucking the other way to the plenium i expected it to be open but is definately blocked, that's why i wondered if the stepper motor shuts it off somehow?
so could the stepper motor be causing the intermittant idle problem or is that too easy?
Thanks again mate, wish i had a memory like yours

I thought my vac was shot after the suck test but once I had taken the dissy off and tried it directly on the "nipple" (yes I know) it was fine, and when I checked it with the timing gun you could see it advance so I dont put much faith in the suck test!

Shud have used the gun first :doh:

Ok, i'll get reet up close on me nipple and give her a go tomorrow

rustyrangie said:

Thanks again kev, just popped home for a coffee
i tried sucking the vac pipe to get the base plate to move, i'm just sucking in air so i guess the bellows are shot??
when i tried blowing/sucking the other way to the plenium i expected it to be open but is definately blocked, that's why i wondered if the stepper motor shuts it off somehow?
so could the stepper motor be causing the intermittant idle problem or is that too easy?
Thanks again mate, wish i had a memory like yours

Click to expand...

Stepper motor is not implicated by what you describe, if there was a problem with the stepper, the idle speed would be either too low or noticeably high.

Where you say you're sucking in air, that could well suggest a damaged diaphragm in the vac unit. Blowing/sucking the other way you say suggests vac pipe is blocked. In that case, remove the pipe and inspect the nipple it connects on to. If the connection is full of crud I would urge you to remove the plenum and clean it thoroughly with carb cleaner.

Had a bit of a poke about today but to be honest i didn't want to get too envolved as i'll be buggered if i break it while the roads are crap

Checked the vac advance again, if i suck with all my might the arm onto the base plate moves about 3mm and to me the base plate feels very stiffly sprung but i have nothing to compare it to
The plenium end is open but again it's not that easy to get air through, should it blow through easily?

Times like this i wish they still made 2 stroke cars

rustyrangie said:

Checked the vac advance again, if i suck with all my might the arm onto the base plate moves about 3mm and to me the base plate feels very stiffly sprung but i have nothing to compare it to

Click to expand...

Sounds fine. Have you looked at/checked the other things I have suggested? Is the ignition timing correct? have you got healthy electrics?

Thanks again Kev
I checked all the wiring, is all good, alternator output was about 13.2v, down to 12.8 fully loaded. I haven't started on the efi sensors yet as i don't want to break owt before the weekend

I haven't checked the timing yet, my strobe's in me workshop miles away. But i checked the dizzy over, the caps fine but the rotor arm looks odd, i haven't seen it for a while so i've forgot what it looks like but the contact tip looks odd. if you look at it from above the tip isn't symetrical, is that right?

rustyrangie said:

Thanks again Kev
I checked all the wiring, is all good, alternator output was about 13.2v, down to 12.8 fully loaded. I haven't started on the efi sensors yet as i don't want to break owt before the weekend

I haven't checked the timing yet, my strobe's in me workshop miles away. But i checked the dizzy over, the caps fine but the rotor arm looks odd, i haven't seen it for a while so i've forgot what it looks like but the contact tip looks odd. if you look at it from above the tip isn't symetrical, is that right?

Click to expand...

MMMMMMM......... sounds a bit odd does that rotor arm. Can you post a pic? Have you looked at the inside of the dizzy cap? are the nodes inside clean or have they got white powdery deposits? Might be worth getting some fine wet & dry and carefully giving them a clean up, same for the tip of the rotor arm which as far as I recall should be symetrical. Is it a genuine Lucas arm or a pattern one?

BTW your alternator output looks a little low, would have thought you'd be wanting at least 13.8v but 14.4v best. What condition are the brushes in? Out of interest do you get a misfire or stumble when engine under load, up to temp with high electrical load?

The reason I ask is that some while ago I used to get odd running from mine and at night with say heater, heated rear screen and then main beam, I would get a misfire. This was caused by a failing alternator and was most noticeable with the engine under load.

i did check the dizzy cap, it's only done about 3k anyhow but was fine, pins were all clean.
Do you see what i mean about the rotor
I just went out and checked the alternator output again, it's putting out 13v from cold on fast idle.
looks like the battery is pooped. I bought one about 18 months ago but it hasn't been holding a charge, i thought it was the alarm draining it as i don't get to use the range very often. i took it off and put a known good one in but she barely turned over just now and has only sat for a day

Sounds like you need to get your alternator sorted. Find a local auto electrician and get them to put in new regulator, rectifier, brushes and slip rings. It shouldn't be that expensive and should be done in a day.

Next, try cleaning up your rotor arm as an interim measure but do get yourself a new one and a spare to go with it. DO NOT buy pattern rotor arms. Genuine ones are relatively expensive but are so totally worth it.

Let us know how you get on.

I'll let you know what happens, i'd only ever get lucas bits, tuthers are ****e..
I guess it's not worth chancing a second hand alternator then?
to be fair i think the one in there is original and done 150k

rustyrangie said:

I guess it's not worth chancing a second hand alternator then?
to be fair i think the one in there is original and done 150k

Click to expand...

Only if you know it is in good working order!! I know all about alternators after last week BTW cheers again to the guys on here who helped me out.

Haven't had a chance to do much yet, had to go back to work
I did buy a new rotor arm, feck me, i dunno what happened to the old one, guess i didn't change it when i did the cap but the contact tip were over 2mm shorter, i guess someone got over eager with the emery paper....
I'm gonna have a dig about tomorow night and do timing etc, i can't afford to do owt with the alternator til i gets paid

I have exactly the same problem on a v8 lpg. Whenever under load starts to struggle and get the odd misfire. If towing mrs' horse trailer the whole thing goes into some sort of cardiac arrest following which hazards go on until I take key out. Changed everything leads plugs etc. I have noticed recently at night the lights flickering which I suspected was an early indication of a fooked alternator. Anyone had similar and is this the case?