Engine trembling/jerking while driving on LPG

[volvojj]

Problem with ignition/spark - leads/cables/wires or maybe something else? – Engine jerking while driving on LPG

Hello,

Last autumn my Volvo 940 (2.3 1995 soft turbo 99KW) started to shake when I would drive the car on LPG (while pressing the gas paddle the engine would jerk and accelerate slower than usual). When I would switch to petrol the problem would disappear. As a result, I continued to drive the car on petrol during the winter.

Two weeks ago I decided to take the car for a checkup/service at the shop that installed the LPG, since I thought that the problem with the engine jerking has something to do with the LPG system. After changing the LPG filters (per my request) and after doing software scan, the mechanic told/showed me the following:
- He used pump to spray the ignition cables with small amount of water. Immediately what we could see were electric jolts between the ignition leads and the engine. He said that this affects the LPG system which is the reason for the engine is trembling. Since there is no trembling when the engine works on petrol I decided to turn off the LPG and drive it on petrol while I find solution to the problem.

The LPG mechanic told me that this problem (the electricity jolts between the cables and the engine) is most probably due to faulty ignition cables and the best thing to do is to change them with new ones.

While this might be the case, - I would like to hear your suggestions what might be other potential reasons for this problem. If somebody had a similar problem was it solved simply by changing the ignition leads or do was it something more complicated? For example, potential problem with the distributor (cap, rotor or moisture barrier) or maybe problem with electricity short-circuit elsewhere in the car/or something similar?

The reason that I suspect that it might NOT be the cables (although I wish it were and I solve the problem easily simply by changing them) is the following - currently I use ignition cables that I bought second from eBay-Kleinanzeigen in Germany and I installed them about a year and a half ago. The guy that sold them to me said that they have only 15.000 miles (they are BOUGICORD-he said he bought them from Skandix) but he switched to MAGNECOR (45231) after half a year. Yesterday I was thinking that he might tricked me and had sent me his old cables, but when I compared them with my original old cables the rubber on his was much newer plus he has sent me 5 cables.

I would appreciate if you have any ideas about what might be the problem or what is the best way to trouble shoot and detect what is the issue?
Kind regards

[classicswede]

I have been installing and running LPG for nearly 30 years and can confirm what your installer is saying that it is likely the ignition system

You want Genuine Bougicord leads (also Volvo or Magnacore will work) genuine Bosch distributor and rotor arm and NGK plugs BPR6ES or 7ES

As LPG is dry it requires a much stronger spark and it highlights any weakness in the system

[Rversteeg]

You do not mention the LPG system you have. If it is a system which injects liquid LPG (Vialle LPi) or gaseous LPG then it could also be one of the injectors failing.
But as classicswede says, the ijnition system has to deliver 20-25% higher voltage to ignite LPG, a weak component will first show up when driving on LPG. I would personally not hesitate to first install new HT leads, as these are wear parts anyway. Don't bother with second hand stuff for wear parts, for about 80 Euro you have a new set of Bougicords. Running on petrol instead of LPG is more expensive on the long run...

[volvojj]

Thanks Classicswede and Rversteeg for your answers. Here are my replays to your posts:

To: Classicswede
- Today I have found local shop where they have NGK BPR6ES (7822) and I should be able to pick them up tomorrow. Furthermore, this is the spark plug that they offer at Skandix as well.
- I have also found a newer NGK spark plug for LPG (1497) as a substitute for BPR6ES. Do you have any experience with it and would you recommend it?

To: Rversteeg
- The LPG system that is installed distributes gaseous LPG. The installer has checked the system on his PC and did not mention potential problem with any of the injectors.
- Is there anyway to identify the week component, whether is the leads, cap, rotor or smth else? Maybe some other web-link that already describes the procedure how to troubleshoot the weak component...
- I agree that one should not save on wear parts, in my case just need more time to organize the order and delivery home - as not all parts are available locally. That is why I started this thread - to hear suggestions how to identify the potential reasons for problem, and then see what will I need to order.
- Most probably I will order new Bougicords from skandix. Locally the best option is NGK 8469 (RC-VL606). Do you know how good are they - having in mind that on most of the forums they recommend Bougicords...

Finally, if I may ask both of you regarding the electricity jolts between the ignition leads and the engine - is this normal? This is what I wrote about what the mechanic did:
"- He used pump to spray the ignition cables with small amount of water. Immediately what we could see were electric jolts between the ignition leads and the engine. He said that this affects the LPG system which is the reason for the engine is trembling."

Kind regards

[Chris1Roll]

Originally Posted by volvojj

Finally, if I may ask both of you regarding the electricity jolts between the ignition leads and the engine - is this normal? This is what I wrote about what the mechanic did:
"- He used pump to spray the ignition cables with small amount of water. Immediately what we could see were electric jolts between the ignition leads and the engine. He said that this affects the LPG system which is the reason for the engine is trembling."

Kind regards

This is not normal and is demonstrating that the insulation on your leads has broken down and the leads need replacing.

[classicswede]

Typically plugs that say for LPG should be avoided. The best thing is just normal plugs with a single prong and just replace more often.

The spraying of water on the cables does show they are past their best. I would also suggest checking the distributor cap and rotor arm as issue there will also cause the problem

A single cylinder misfire, you can move the cylinder a injector feeds to see if the misfire stays or moves to prove the function of them. Your issue sounds more randon and over more than 1 cylinder. Fix the ignition problems first before trying anything with the LPG