Engine started running rough - Help Please

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jt_armstrong

jt_armstrong

Well-Known Member
Posts
1,824
Location
Lake Forest, California
Was out for a run earlier when I noticed that the engine tone had changed. It appear to me and the wife that the car had got a little throatier. Unfortunately it also felt like there was now also an imbalance like she was running on one less cylinder. I had a 30 mile drive home and the whole way I was convinced that I would be coming to change out the spark plugs or maybe something worse.

I pulled all the plugs and they all looked perfect. Nice gold colour with a few deposits. There was certainly no black oily ones which was what I expected to see if the cylinder was not firing.

I put them all back in but she still sounds sick. No codes or lights are showing on the dash.

Any ideas?

Thanks
John

P38 1997 4.6L HSE
 
try unplugging the mass air flow meter,,, get someone to keep the revs up for you then unplug it if it runs fine then you need a new maf,,, dont be temted to get a cheap one they dont last...
 
robp38 said:
try unplugging the mass air flow meter,,, get someone to keep the revs up for you then unplug it if it runs fine then you need a new maf,,, dont be temted to get a cheap one they dont last...
Click to expand...

OK, so I disconnected the plug which goes into the Sagem unit (assuming this is the MAF sensor). The wife was holding the revs at 2000rpm and when I pulled the plug nothing changed with regards to engine speed. I did the same thing at idle speed and again nothing happened. What should happen?
 
rewmer said:
Treat her to a new set of BPR6ES plugs and a new set of leads and go from there. Don't get to caught up in deep diagnostics just yet.
Click to expand...

Would you not see evidence of failing plugs and leads on the actual burn surface of the plugs? My plugs looked perfect. I also took the MAF sensor off and gave it a good clean. When I disconnect it now at idle she almost stalls but recovers to real lumpy idle.

She has run beautifully since I got her and has done almost 10K miles with out any real problems. Whatever happened this afternoon was sudden and not gradually over time. The only thing that I did today was to fill her up but that was about 20 miles before things went wrong.
 
It may well be coincidence but you filling up 20 miles before does sound a little ominous. Bad fuel / water in the fuel? Was it supermarket or known brand?
The sounding 'throatier' would also indicate something either blowing or sucking that shouldn't be. It doesn't take much of a leak on the exhaust, especially near the manifolds or cats, to knock the performance down and make it sound wrong. If one of your cats had a problem, this would affect the fueling and therefore how the negine was running. Perhaps an emmissions test may throw some light on it or a diagnostic check by someone who can correctly interpret the figures. If your plugs look OK, I'd say the coil packs and leads probably are too. You'd know if it was missing by the 'feel' more than the sound. It doesn't make it sound 'throatier' though. Good luck.
 
Derek Ward said:
It may well be coincidence but you filling up 20 miles before does sound a little ominous. Bad fuel / water in the fuel? Was it supermarket or known brand?
The sounding 'throatier' would also indicate something either blowing or sucking that shouldn't be. It doesn't take much of a leak on the exhaust, especially near the manifolds or cats, to knock the performance down and make it sound wrong. If one of your cats had a problem, this would affect the fueling and therefore how the negine was running. Perhaps an emmissions test may throw some light on it or a diagnostic check by someone who can correctly interpret the figures. If your plugs look OK, I'd say the coil packs and leads probably are too. You'd know if it was missing by the 'feel' more than the sound. It doesn't make it sound 'throatier' though. Good luck.
Click to expand...

Thanks Derek
I'm beginning to lean towards fuel being the problem. It was a shell garage so would have thought it would be OK but they did me a leaflet saying something about their new fuels giving you more MPG.
 
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