UKZR1

Hi guys, I need some help!!

My Z developed a misfire a while back after a trip of about an hour at around 3-figure speeds - it was noticed after slowing on some town roads and then the rest of the way home. First thing I changed (as it was the
easiest) was the plugs - this appeared to help at first but again after a trip of over an hour the misfire had returned.

The plug wires were original and at 15 years old a probable cause. Checking the resistance of each pair of plug wires (through the coil) and comparing the readings to some obtained from doing the same on another car made me suspect the wires even more so - the readings on 1 pair would fluctuate massively when the wires were moved a little.

Time to pull the plenum.

Plenum pulled old wires thrown, old plugs out and new wires and plugs to go in. Removed all 4 coil packs - some sign of corrosion on the spade connectors they sit on so cleaned all that up and then checked the resistance of the primary and secondary coils on all 4 packs. The readings were identical on each pack. Re-assembled everything cleaning all connections as I went and checking all vacuum lines as well - all good.

With it all back together I fired it up and the engine appeared to run real well - ticking over noticeably smoother than before (even the wife
noticed ) With a trip into Europe just around the corner I took the car on a test run and all appeared to be fine, misfire gone making those last couple of evenings in the garage all worthwhile.....I thought

Just into Europe the misfire returned again

A couple of points here -

The engine always feels fine when cold (say for first 1/2 hour) - make me think its temp related

The gauges all read normal T & P's but everything in the engine bay feels way too hot. After returning from Europe last night, I couldn't even touch the plastic oil cap!! The plenum is very hot (too hot to
touch) as is the upper coolant tank. Now I'm not sure if this is due to the misfire or if its causing the misfire.

If I accelerate slowly (put my right foot down slowly - that's so hard!) it will maybe be ok. If I put my foot down quickly it will misfire much worse. It feels worse at around 2000rpm although I'm sure its the same throughout, its just more noticeable at this point.
It is ticking over to slow and real lumpy so must be missing at idle as well.

Any ideas???

thanks
Brett(UK)
www.corvetteclips.co.uk

Jeffvette

Heat soak on the packs would be my best guess. Cool and low operating temps they do fine. Hot underhood temps build up rather quickly.

ZR1forFun

Brett,

Is the ECM throwing any fault codes?

Give me a call this week and we can try and troubleshoot it.

RatRacer

Bad injector(s)?

UKZR1

No fault codes at all Richard. Tried running with your chip and Jeals as well, no difference. Even tried turning the power key off and still the same.

It's hotter in the engine bay than normal - I wonder if thats because it is running lean and that is causing the misfire.

90 Corvette ZR-1

that's what I'm thinking

UKZR1

Thanks for the help so far. I've had a few suggestions as what to check, bad fuel injector being the most common. I will start trouble shooting this weekend and will let you all know how it goes.

LoTu5

Brett,this sunday i am going to visit my parents,so the following weekend will be free,also i will have some more plenum gaskets in stock,if you want to drive down then,pull both plenums and swap parts around to trouble shoot feel free,you could then help/show me how to set up the TPS

Cliff

UKZR1

Thanks for the offer Cliff but I will be on-call for 7 days starting on Monday so next weekend will be a no-go. Perhaps another weekend in October...Damm its a shame all you guys are down there.

Another option would be for you to take a drive up here if its driveable by then. Perhaps have another get-together. I'm sure Richard would be up for that as well.