spannersNK
New Member
Hi All, hoping somebody can shed some light on this it's driving me nuts!
I have a '96 4.6 p38 with BRC LPG and a Tornado chip. The liners went at 92,000 and so I rebuilt the engine with a Turner Engineering short engine with a H180 Cam, it started first turn of the key and ran like a sewing machine for about 8,000 miles. One day I noticed the idle was very slightly rough, causing a bit of vibration when sitting in traffic, now whether it came on suddenly or gradually I'm not 100% sure but I'm pretty good at noticing that sort of thing so I think it was most likely suddenly.
I've been trying to sort it for around 5000 miles now with no real success. It's barely noticeable and as I say more annoying than anything else as you really feel it when stationery, seats and steering wheel vibrate and because I know it was and should be super smooth it really irritates me!
It's got no worse or better over the 5000 miles and as soon as the revs go above about 800/850 you don't feel it but I can still just hear the miss through the exhaust. It's very much a regular miss/cough ever half second or so at idle and less frequent as the revs rise. Power and general running is otherwise perfect.
It's the same on petrol or LPG but you do notice it a bit more on LPG I guess because it's more sensitive than petrol.
I started with the basics, plugs and leads were a changed, plugs good colour bit worn, one Magnecor lead a bit cracked and they were 35,000 miles old so changed them as a precaution.
Compression 175-180 on all cylinders
No difference.
So far:
1. Went to a local LPG chap for some diagnostics, he tried a new MAF and suggested changing O2 sensors in case of intermittent fault, no change
2. Faultmate shows no error codes and I think all readings are within range as far as I can ascertain.(screenshot attached) Reset adaptive values 3 times after component changes.
3. Changed the idle air control motor as it was sticking in one direction.
4. Checked for vacuum leaks, thought I'd found slight one at the drivers rear end of the manifold, brake cleaner made idle rise very slightly and when checking the coils one rear manifold bolt was loose so rebuilt the top end with new valley gasket etc as a precaution.
Again no change, vacuum shows 17in, bit low on the gauge but is this a normal value? I have a H180 cam in it.
5. One coil pack showed very slight low resistance so changed it to rule it out.
6. checked crank sensor and reluctor ring, one tooth slightly bent, fixed it, no change. (am I right in thinking this either works or doesn't) Once it wouldn't start for about 20 mins after having the ignition on for a while while running fault mate and sometimes the idle will drop to around 450/500, can the CPS cause a rough idle?
7. Cleaned my MAF have tried a working secondhand one as well, car runs without MAF after a bit of revving but still same miss at idle without it so figure it can't be that and don't want to just fork out for a genuine one without good reason (cheepos are not advised from what I read). Airflow varies between 25 and 28 at idle (760rpm) bit above spec of 22 but voltage is 1.67 at idle rising to around 2.8 at 3000 so think this indicates it's functioning?
8. Have disconnected injectors one at a time and each results in a different constant miss,plus as is the same with LPG can't be that?
All of this has improved the mileage a bit but not the misfire.
Have pretty much run out of ideas now, don't think it can be any major internal problems as it so new and has no bad noises, plus it came on suddenly and has not got worse in 5000 miles with off idle running still 100%
Any ideas? Anybody know a Rover genius in the SE/Kent area as it's driving me mad!
Thanks in advance.
Will.
I have a '96 4.6 p38 with BRC LPG and a Tornado chip. The liners went at 92,000 and so I rebuilt the engine with a Turner Engineering short engine with a H180 Cam, it started first turn of the key and ran like a sewing machine for about 8,000 miles. One day I noticed the idle was very slightly rough, causing a bit of vibration when sitting in traffic, now whether it came on suddenly or gradually I'm not 100% sure but I'm pretty good at noticing that sort of thing so I think it was most likely suddenly.
I've been trying to sort it for around 5000 miles now with no real success. It's barely noticeable and as I say more annoying than anything else as you really feel it when stationery, seats and steering wheel vibrate and because I know it was and should be super smooth it really irritates me!
It's got no worse or better over the 5000 miles and as soon as the revs go above about 800/850 you don't feel it but I can still just hear the miss through the exhaust. It's very much a regular miss/cough ever half second or so at idle and less frequent as the revs rise. Power and general running is otherwise perfect.
It's the same on petrol or LPG but you do notice it a bit more on LPG I guess because it's more sensitive than petrol.
I started with the basics, plugs and leads were a changed, plugs good colour bit worn, one Magnecor lead a bit cracked and they were 35,000 miles old so changed them as a precaution.
Compression 175-180 on all cylinders
No difference.
So far:
1. Went to a local LPG chap for some diagnostics, he tried a new MAF and suggested changing O2 sensors in case of intermittent fault, no change
2. Faultmate shows no error codes and I think all readings are within range as far as I can ascertain.(screenshot attached) Reset adaptive values 3 times after component changes.
3. Changed the idle air control motor as it was sticking in one direction.
4. Checked for vacuum leaks, thought I'd found slight one at the drivers rear end of the manifold, brake cleaner made idle rise very slightly and when checking the coils one rear manifold bolt was loose so rebuilt the top end with new valley gasket etc as a precaution.
Again no change, vacuum shows 17in, bit low on the gauge but is this a normal value? I have a H180 cam in it.
5. One coil pack showed very slight low resistance so changed it to rule it out.
6. checked crank sensor and reluctor ring, one tooth slightly bent, fixed it, no change. (am I right in thinking this either works or doesn't) Once it wouldn't start for about 20 mins after having the ignition on for a while while running fault mate and sometimes the idle will drop to around 450/500, can the CPS cause a rough idle?
7. Cleaned my MAF have tried a working secondhand one as well, car runs without MAF after a bit of revving but still same miss at idle without it so figure it can't be that and don't want to just fork out for a genuine one without good reason (cheepos are not advised from what I read). Airflow varies between 25 and 28 at idle (760rpm) bit above spec of 22 but voltage is 1.67 at idle rising to around 2.8 at 3000 so think this indicates it's functioning?
8. Have disconnected injectors one at a time and each results in a different constant miss,plus as is the same with LPG can't be that?
All of this has improved the mileage a bit but not the misfire.
Have pretty much run out of ideas now, don't think it can be any major internal problems as it so new and has no bad noises, plus it came on suddenly and has not got worse in 5000 miles with off idle running still 100%
Any ideas? Anybody know a Rover genius in the SE/Kent area as it's driving me mad!
Thanks in advance.
Will.
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