JasperV8

New Member

• Jul 19, 2020

• #1

Hi anyone out there have any advice where I should start with the following issue.

Up to a few weeks ago no issues but now particularly on LPG but present on petrol too, when under load I get poor power and what sounds like popping like a back fire coming from the back of the engine bay.

Its done about 25k miles since the last change of plugs so was planning on this first.

No DTC s present for the engine.

I am no motor tech, just try and do a bit myself if I can, so excuse my poor description.

Any advice welcomed, thanks.​

 

DanClarke

Well-Known Member

Full Member

• Jul 19, 2020

• #2

HT leads & coil packs ok?

 

Henry_b

Think outside, no box required.

• Jul 19, 2020

• #3

Check the plugs, poor ignition on LPG will cause it to backfire through the manifold..

 

JasperV8

New Member

• Jul 19, 2020

• #4

DanClarke said:

HT leads & coil packs ok?

Click to expand...

Just checked them and cant get on lead off, one plug has quite a bit of blackening on the top contact, and one lead will not go back on very well the contact has pushed back in the grommet, so I guess I will change the leads. Hawkeye now showing misfires. Is it worth changing the coil packs as well?

 

DanClarke

Well-Known Member

Full Member

• Jul 19, 2020

• #5

My LPG multipoint system on my V8 would feel decidedly under-powered and a certain "lumpiness" could be detected. On each occasion replacement of a faulty HT lead or Coil-pack cured the issue.
LPG in particular is VERY sensitive to HT issues.

 

JasperV8

New Member

• Jul 19, 2020

• #6

DanClarke said:

My LPG multipoint system on my V8 would feel decidedly under-powered and a certain "lumpiness" could be detected. On each occasion replacement of a faulty HT lead or Coil-pack cured the issue.
LPG in particular is VERY sensitive to HT issues.

Click to expand...

Thanks, I will get new set of leads and coil packs and swap them out.

 

DanClarke

Well-Known Member

Full Member

• Jul 19, 2020

• #7

JasperV8 said:

Thanks, I will get new set of leads and coil packs and swap them out.

Click to expand...

May not be your issue, but it is a common one with LPG & the V8.
Wait a while and see what other suggestions come forward from the forum crew.

 

R

Ratae

Well-Known Member

• Jul 19, 2020

• #8

With the exception of a misfire at high revs with a pattern dizzy cap the misfires I have had on LPG on mine (86 RRC 3.5EFi with BLOS LPG carb) have been a plug. Two occasions in around 20 years.
I view plug leads as a service item too. I use standard decent quality ones, never felt the need for magnecore or such.
Replace the stuff you know is defective & see what happens.

 

JasperV8

New Member

• Jul 23, 2020

• #9

Thanks for the info guys, so replaced leads and plugs but the plugs fitted were non standard Denso W16EPR-U not the Champion RC11 PYP B4 as in the manual, this looks like maybe switched out when LPG conversion was done, any comments on plugs you guys are using in LPG 4.0 V8's?

Any way swapped all leads and plugs out(managed to do it without taking the IM off) had to use NGK equivalent of the Denso ones that were in there and back fire is gone, but now missfire has moved to cyl 7 from cyl 3, also have fault 112 P0304 cam shaft position not plausible with rough running, should I now change the cam shaft position sensor?

 

R

Ratae

Well-Known Member

• Jul 28, 2020

• #10

Swap plugs 7 & 3 & see if the misfire follows the plug. If it doesn't do the same with the leads.
Can't help with the cps issue.

 

kermit_rr

Very

Full Member

• Jul 28, 2020

• #11

Have you adjusted the plug gap? Needs to be slightly smaller for lpg

 

R

Ratae

Well-Known Member

• Jul 28, 2020

• #12

^^^
Did you check the plug gaps before fitting the new plugs to be sure there was one?
Remember a competition for young mechanics which included fitting a set of new plugs - which were BNIB.
The judges had closed one plug's gap completely.
I can't recall whether it was two or just one competitor that just fitted the new plugs without bothering to check them at all.
They then spent a VERY long time trying to find the misfire before deciding to recheck the plugs......

 

JasperV8

New Member

• Jul 28, 2020

• #13

Thanks for all your advice guys very helpful. Yes did check plug gaps. Tried swapping plugs about and leads and misfires still present on 7 with the odd appearance of 3 an 8. Changed out coil packs, did have to take the IM off for this, and if I had not had a magnet I would have been in real trouble. Running sweet now so looks like it was the coil packs after all. Cheers again guys.

 

R

Ratae

Well-Known Member

• Jul 29, 2020

• #14

Fingers crossed!