VR Caprice Stock RPM Tachometer Bouncing / Engine Running Normal

[vr84]

** NOTE TO FUTURE READERS ***

Problem has been resolved, it was the ignition module, can read below the problems that occurred in the lead up to this.

** Original Thread Below **

Hi everyone, wondering if anyone can help me out on this problem.

My RPM tacho had always worked fine until a recent spark plug change and starter motor swap. Since then the gauge is bouncing around however the engine is running 100% normal on petrol.

The gauge will bounce from the actual RPM to about 1000 - 2000rpm above that, then quickly drop back down. This is a new problem that never occurred in the past.

I also have dual fuel, and since this occurred the LPG system is totally out of whack, it can't properly detect the RPM and is cutting out constantly assuming the engine is either turned off or it can't detect the RPM to switch from Petrol to LPG. As such I have been running entirely on petrol recently.

Runs fine on petrol, engine is perfect with no misfires too, just the tacho going crazy.

I'm hoping to find a lead to the tacho problem and try to sort this and see if the LPG then comes back to normal too. Previously the car was running perfect on both fuels and RPM tacho was 100% accurate.

Any help is much appreciated

 

[BlackVXGTS]

The taco feed comes from the ignition module/ignition coil. What type of spark plugs are you using now, and are they the same type as you replaced (including gap)?

 

[UTE042_NZ]

1st guess is a miswire. Something plugged back in wrong, not plugged back in, shorting, loose connection, and/or all of the above.


"recent spark plug change and starter motor swap"

 

[CCCMikey]

When I programmed my new LPG computer, it had a setting for RPM type, and I had to choose the "Weak" option.

I still couldn't do the auto config because the once I get over 3,000 RPM, the LPG computer registered it between 4,000 and 2,000 randomly, a bit like you're seeing; except it only affected the LPG computer but the tacho on the dash is normal.

For the LPG system, I just programmed similar injection time factors irrespective of RPM over 3,000 - but I rarely hit that speed anyway.

How are your gear changes / TCC Lockup reacting? That might help diagnose whether it's just the tacho being a bit silly, or if the computer itself thinks that's what happening.

Might be worth pulling the fuse on the LPG system too, just to see if it makes any difference.

Only other thought is to check that the battery terminals are all connected OK - including the LPG computer's additional connections if there are any.

 

[vr84]

Thanks for your help, from your advice I've taken a look over the car and came up with the below theory

The taco feed comes from the ignition module/ignition coil. What type of spark plugs are you using now, and are they the same type as you replaced (including gap)?

Identical including Gap

1st guess is a miswire. Something plugged back in wrong, not plugged back in, shorting, loose connection, and/or all of the above.


"recent spark plug change and starter motor swap"

I'm starting to think it may be the LPG wire that runs to the coil for RPM, maybe it's come loose or touching one of the leads to get a false signal sent to both the LPG and tacho.

When I programmed my new LPG computer, it had a setting for RPM type, and I had to choose the "Weak" option.

I still couldn't do the auto config because the once I get over 3,000 RPM, the LPG computer registered it between 4,000 and 2,000 randomly, a bit like you're seeing; except it only affected the LPG computer but the tacho on the dash is normal.

For the LPG system, I just programmed similar injection time factors irrespective of RPM over 3,000 - but I rarely hit that speed anyway.

How are your gear changes / TCC Lockup reacting? That might help diagnose whether it's just the tacho being a bit silly, or if the computer itself thinks that's what happening.

Might be worth pulling the fuse on the LPG system too, just to see if it makes any difference.

Only other thought is to check that the battery terminals are all connected OK - including the LPG computer's additional connections if there are any.

Checked the battery terminals and all seems ok, the work on the starter motor shouldn't have affected anything, but the plugs change i noticed the leads aren't separated via the plastic clip anymore, so ordered a separator to see if that is the issue.

There is one wire running from the LPG computer to the coil though that looks like it's a fairly dodgy setup, its fairly loose and runs along the firewall to the coil, maybe when the leads came off and went back on, one of the lead wires came into contact with this wire and the spark is jumping between the lead and this wire, which should probably explain the situation.

Planning to take it to an LPG mechanic in a few days to check out if anything is interfering with this wire running from LPG to the coil. In regards to the engine on petrol it runs fine, but i have noticed the engine misfires a tiny bit on idle when the tacho is going weird, but aside from that runs perfect on petrol, hence my thoughts it's most likely something to do with the coil and LPG RPM signal.

Gearbox shifting is fine on both petrol and LPG.

Position of the coil in this car though is a nightmare to get to, so I can't see visually if there is anything wrong, so job for the mechanic.

 

[vr84]

Today it began to misfire on both petrol and LPG, it's at the mechanics now, will update with the issue soon

 

[woteva]

Today it began to misfire on both petrol and LPG

From working on LPG conversions many years ago, I do remember that a lot of running problems would show up on gas before petrol.
We had customers drop off their car for gas conversions and be upset we had to replace tune up parts to get it running correctly. They would always say it was running fine when they dropped it off

 

[Immortality]

Have you made sure all the leads are securely fitted on the plugs?

Can you borrow a known good coil to test with as well as the ignition module?

Check and clean all earth points and go over everything you touched doing plugs and starter.

 

[vr84]

Mechanic changed leads, minimal improvement, most likely the distributor.

LPG will be removed after this as it had the module, coil and distributor replaced with new / change over parts 2 years ago. Seems the LPG is just burning through the ignition system, it's always used new bosch parts too.

No savings on LPG while it's causing problems like this, best to bin it and stick to petrol.

 

[Immortality]

I'm no LPG expert but as I understand it LPG is harder on ignition systems because it is harder to ignite and hence the reason when running LPG you run a slightly smaller spark plug gap.