Hi all,

Seeking for help with a very frustrating shuddering issue I've been having, with no luck in isolating what could be the issue. Car is a 03 TJ S2 4sp auto magna with lpg. Was previously manual.

Problem: Car severely shudders during acceleration from 2000rpm onwards. It is very strong and rattles the whole car. It's not dependent on if the car is turning or driving straight. Shuddeirng Can be felt through steering wheel. Shuddering disappears past 6,000km.
Not sure if related but the engine is very rough when it's warmed up and stalls at low revs. Engine is settled on cold.

Things I've done/checked:
- All engine and gearbox mounts replaced with new units.
- Checked for freeplay in wheels/hub. None found. No vertical or horizontal play when steering is locked.
- Control arm bushes checked. No signs of damage. Car has no noises over bumps.
- Strut tops replaced with brand new KYB ones. Brand new suspension throughout.
- Passenger driveshaft replaced 40,000kms ago. No clicking while turning and no visible fault present.
- Drivers side driveshaft replaced with same brand as passenger one but the cv joint dislodged after a few months of use. Even replaced intermediate shaft bearing with new unit. No change. Driveshaft assembly now fitted with good condition factory driveshaft.
- IridiumIX Spark plugs all checked - looking healthy.
- Leads replaced with both 2nd hand good condition factory leads and brand new aftermarket bosch leads.
- Distributor rotor and cap in perfect condition. Dizzy itself swapped out with 2 other 2nd hand units for testing purposes; No difference.
- Manual gearbox removed and fitted with auto. No change. (Auto was put back in for other reasons than just to solve shuddering issue).
- Vacuum test with vacuum guage plumbed into fuel reg vac line. Result was a steady 20kpa.

I'm getting a bit concerned that with the rough idle on warm issue that I should be looking at the engine for an internal issue?
I'll need to get a compression test done.

Anyone have any ideas/suggestions what could be the cause?
What else should I look at?

Appreciate any and every help! I'm going mad over this issue.

Thanks,
MagnaP.I

Did you notice if this problem was more or less apparent when you changed driveshafts?

I had a slight shuddering problem that was noticeable (to me) when accelerating from about 80kph to 120 kph on smooth straight country roads. (It disappeared when the car was cruising.). It was most obvious in the hills when going up slight inclines. I thought it was 'missing' caused by plugs, fuel, or some other engine problem. Turned out to be CV joints. When the joints were replaced the shuddering disappeared. So my point is that dodgy joints can cause problems at higher speeds in a straight line, not just when going slowly around corners.

Does it shudder when you run the car on petrol or does it only do it when running on LPG ?

Thanks for the responses :)


Did you notice if this problem was more or less apparent when you changed driveshafts?

I had a slight shuddering problem that was noticeable (to me) when accelerating from about 80kph to 120 kph on smooth straight country roads. (It disappeared when the car was cruising.). It was most obvious in the hills when going up slight inclines. I thought it was 'missing' caused by plugs, fuel, or some other engine problem. Turned out to be CV joints. When the joints were replaced the shuddering disappeared. So my point is that dodgy joints can cause problems at higher speeds in a straight line, not just when going slowly around corners.

To be honest, nothing changed with the change of driver side driveshafts. The shudder has been there for a long time but now it's worse than ever and unable to be ignored. This isn't a slight shudder; the entire car badly rattles.

The passenger driveshaft was replaced about two years ago so maybe the cheap $100 shaft I bought may be cactus. Might pick out a 2nd hand unit for testing purposes.


Does it shudder when you run the car on petrol or does it only do it when running on LPG ?

There's shudder on both fuel sources. Petrol seems to be more noticeable though.

Sounds like you need to eliminate the bad idle/stalling issue before looking at drive-line. Does the engine shake and run rough when revved in neutral?

I'd like to isolate that issue but was really wanting to get rid of the shuddering issue first. Maybe they are related.

I might need to accelerate with the wheels off the ground and see if the engine shakes unnaturally and badly during the acceleration.

I had similar issues with my old 2001 tj 4spd fwd and it turn out to be the front diff and gear box,we had to get used gear box fitted and it went away.

Thanks but as I said in my first post, the shudder existed when I had a manual gearbox. The auto was fitted last week and the car still shudders so it can't be related to the diff. (Car is fwd).

My point was that the entire shuddering issue could be engine related and nothing to do with the driveline, with the bad idle and stalling being other symptoms of the same issue. If the engine revs rough in neutral that would bear this theory out, unfortunately if it's smooth it doesn't necessarily mean driveline since the issue might only appear under load.

Thanks for clarifying that Ensoniq5 :)

I'm looking to the engine now as well. I've crossed off looking too much at the driveline especially since I've replaced parts and checked others.

I have done some investigative work on the engine on the spark plugs, leads, dizzy rotor & cap and even swapped out distributors to try and see if there is any change but got nothing.
Done a vac test and the results were good with no indication if anything was really badly wrong internally but these symptoms are making me think otherwise.
I was thinking maybe a sensor along the line is giving bad readings to the ecu but I've checked for any error codes by grounding Pin 1 of the diag connector and all I had was a continuously flashing CEL light, so sensor is know to be faulty by the ecu.
No much else is left apart from maybe a fuel supply issue. How would I test this I don't know. Might be time to clean out the injectors ? Might see if I can remove the fuel rail as well and have that cleaned out.

Any other suggestions on what to look for?

Sometimes on air valve mixer type dual fuel cars the fuel changeover solenoids in the engine bay do not seal properly when fuel changeover occurs. This can lead to the engine being supplied additional fuel through the leaky changeover solenoid which in turn may cause over fuelling of the engine. On the LPG tank there is a valve which can be shut to isolate the LPG fuel from the rest of the LPG system. Try closing the LPG tank isolating valve then run on LPG to remove residual LPG still in the system then changeover to petrol to see if the problem persists.

Could be a buckled rim - might be worth changing the wheels over to see if that changes anything.

Sometimes on air valve mixer type dual fuel cars the fuel changeover solenoids in the engine bay do not seal properly when fuel changeover occurs. This can lead to the engine being supplied additional fuel through the leaky changeover solenoid which in turn may cause over fuelling of the engine. On the LPG tank there is a valve which can be shut to isolate the LPG fuel from the rest of the LPG system. Try closing the LPG tank isolating valve then run on LPG to remove residual LPG still in the system then changeover to petrol to see if the problem persists.

Do you think that would cause the engine to shudder?
If it was being overfueled by LPG, then wouldn't the engine be fine on lpg then?
I get shuddering on both fuel sources :(

I will shut the valve for good measure.

Really appreciate the advice :)


Could be a buckled rim - might be worth changing the wheels over to see if that changes anything.

Thanks Nem, but I've had different wheels on the car in the past and there was still a shudder. Might put my 3rd set of wheels on and see if there's any differece :)

[QUOTE=MagnaP.I;1606759]Do you think that would cause the engine to shudder?
If it was being overfueled by LPG, then wouldn't the engine be fine on lpg then?
I get shuddering on both fuel sources :(


My sons dual fuel wagon had a similar problem where both fuels were mixing at the same time because both of the fuel solenoids (LPG & Petrol) were allowing fuel to leak past & enter the inlet manifold. You may also have a leaky petrol injector which is allowing some petrol to dribble into the inlet manifold - the fuel pump (petrol) usually remains running when on LPG with the injectors (petrol) not opening as they receive no signal due to the vehicle running in LPG mode. The same goes with the LPG solenoid if it's leaking then additional fuel will be entering the inlet manifold when the engine is running on petrol. If this is the case then the engine/ECU(s) will be going WTF how do I manage/burn 2 fuels with different characteristics at the same time.

My sons dual fuel wagon had a similar problem where both fuels were mixing at the same time because both of the fuel solenoids (LPG & Petrol) were allowing fuel to leak past & enter the inlet manifold. You may also have a leaky petrol injector which is allowing some petrol to dribble into the inlet manifold - the fuel pump (petrol) usually remains running when on LPG with the injectors (petrol) not opening as they receive no signal due to the vehicle running in LPG mode. The same goes with the LPG solenoid if it's leaking then additional fuel will be entering the inlet manifold when the engine is running on petrol. If this is the case then the engine/ECU(s) will be going WTF how do I manage/burn 2 fuels with different characteristics at the same time.

Thanks for the really comprehensive explanation. My lpg system starts on petrol so I assume my fuel pump is always even on lpg.

That makes sense and I can definitely see how dual sources of fuel at once would cause absolute chaos for the engine.

I'll shut the valve and see if there's any difference. Fingers crossed!

Is the problem car speed or engine speed related? To check, get it shuddering (in say 2nd gear) and then change to top gear and see if it still the same. If it is, then it is not engine related and you can look elsewhere. If it always comes in at xxxx R/min, or at a certain throttle opening then that gives you a lead to follow. Does it do it cold or only when hot - another lead. Try to narrow down the potential areas to diagnose.

Well Magna Diver, I shut off the lpg valve and nothing changed on how it runs on petrol. Still shudders and almost stalls when stationary.

To confirm, the shudder occurs whenever I accelerate. No shudder when crusing without acceleration or when in neutral, but only when accelerating.


Is the problem car speed or engine speed related? To check, get it shuddering (in say 2nd gear) and then change to top gear and see if it still the same. If it is, then it is not engine related and you can look elsewhere. If it always comes in at xxxx R/min, or at a certain throttle opening then that gives you a lead to follow. Does it do it cold or only when hot - another lead. Try to narrow down the potential areas to diagnose.

I believe it's engine related. Speed seems to be irrelevant to the shuddering. If I accelerate past 2000rpm, it will shudder. I've changed through gears at 70-80km (to be able to go from 4 to 2 and back) and the shudder is the same. I've checked the driveline visually and can see no areas for concern.
Shuddering happens when cold or hot, but much worse during hot. When the engine warms up I also have idling issues with the car almost stalling at a standstill.

Have you tried running the car on LPG with the fuel pump (petrol) turned off?

Have you tried running the car on LPG with the fuel pump (petrol) turned off?

I haven't because my lpg system uses petrol to start. I'll remove power to the pump under the seat and see if it'll even start.

So have you checked yet if the injectors are leaking?

Sounds like it might be getting too much fuel.

If you have a vapour injection system similar to mine I can start from cold on LPG by doing the following:

1. Press & hold down manual changeover button (with ignition off) on LPG fuel gauge for 30 seconds;
2. Whilst holding the button in turn ignition on & start engine.

Alternatively drive on petrol till it does the auto change to LPG then disconnect power to the petrol pump.

Well I've finally had a chance to spend a bit more time on the troublesome magna.

I did disconnect the electrical connector to the fuel pump and there was no change in both the terrible idle and shuddering during acceleration.

It seems the issue is not fuel related.

Did notice during driving it today that when I would accelerate strongly, I could clearly hear a high pitched squeal like a slipping belt.
I have checked the belts when car was off and they were very tight - no freeplay could be felt. They are in a bit average condition but nothing I'd blame the shuddering for.

I'm beginning to strongly think the harmonic balancer could be busted. I've done a few transmission and engine mount changes with the engine was always attached its mount on the drivers side so maybe one time the balancer cop'd an accidental knock on the chassis rail when I was lowering/raising it.
How could I test the harmonic balancer? Just visually while it's running?

The fear for me is that if it is the harmonic balancer then I'd need to get it replaced very promptly, as it could cause engine damage; if it hasn't already done so.

Do a visual inspection whilst the engine is idling to check for any run out/wobble on the harmonic balancer drive belt face. With the engine shutdown check the retaining bolt is tight & look for any signs of damaged rubber between the two halves of the balancer. Try marking the 2 halves of the balancer with liquid paper / marker pen then give the engine a rev till you hear the high pitched squeal then shut the engine down & check to see if the marks are still aligned. If not aligned then you know the outer section of the balancer is slipping under load.