GTA1990

iTrader: (1)

hi team. Hoping for some thoughts on what might be causing a rough running state after a relatively mild exhaust upgrade

L31 lt4 hot cam with a pro flo xt and headman shorties to a now modified 3 inch y-pipe.

was running the stock headman 2.5 inch y-pipe and no cat but cut and had welded a 3 inch pipe and added a magnaflow cat (also 3”). This all connects to an unchanged 2.75”intermediate pipe and back to a 3” inlet magnaflow back box. Frankenstein exhaust but has all the proper reducers etc. and can’t hear or feel exhaust leaks

So had started dialling in the tune (ebl and a lc2 wideband) and flipped it to open loop on the 2.5” y pipe with no cat setup. Was running decent enough to get a UK MOT which includes an emissions test

now with the 3” y-pipe and cat it’s misfiring and dying at idle and wideband is showing around 16 afr. Hard start aswell. Wideband position on exhaust (just after the 2 to 1 merge on the y-pipe) unchanged and recalibrated the wb in open air before reinstalling

Will be adding some more fuel when I’m back working on the car but just strikes me as odd that a slightly larger exhaust (somewhat countered flow wise by a cat) would have such an impact on the afr, and especially at idle. Or flipping it, might this be a blocked cat?

will report back on my end post fettling this weekend but thoughts ahead of that welcome

ULTM8Z

Possible exhaust leak upstream of the O2?

GTA1990

iTrader: (1)

I won’t rule it out and might explain the leaner afr but perhaps not the rough running (given it’s running open loop). Headers to engine are certainly snugged up with some new fel pro gaskets but yep I can double check the collector side

ULTM8Z

I'm assuming you removed the spark plugs during any removal/re-installation of the headers? If not, possible accidental cracked spark plug? Are any plug wires touching the headers?

GTA1990

iTrader: (1)

thanks Ultm8z and fair. I actually had the spark plugs out and distributor off for a recent oil pan change. I did check these (and rechecked the position of the distributor and timing) including wire clearance from headers but worth a recheck with a fresh pair of eyes.

ULTM8Z

After tracing a high fuel pressure issue to a collapsing vacuum line, nothing is now too ridiculous to check IMO.... lol

GTA1990

iTrader: (1)

Right spent some time with the car today. Of all things, turned out the map sensor had come loose. It’s a TPI sensor that I’ve sort of rigged to the after market intake. EBL was showing 93kpa with it off. On firing up it would stick at 93, trigger the hi map ses light and then eventually die. Vac gauge was showing around 17 which tripped me to thinking no vacuum leak. Removed the map, double checked wiring etc and then refitted. Then was showing 100kpa key on engine off and fired up straight away. Will revisit my installation!

i did pull the cat but made no difference (hot cam with an open y-pipe sounds pretty serious btw).

so yeah, a curve ball because of some custom work by me. Anyway glad it’s finally moving forward and can get it on the road (just in time for the low emissions charge in Greater London..,).