How to diagnose a random misfire?

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How to diagnose a random misfire?

Postby c4mo » Tue Mar 19, 2013 6:27 pm

The problem im having is with a sbc tpi in a 1985 corvette. The problem happens around 3500-3800 rpm and it will skip back and forth, otherwise the engine is fine. Also this is under load only it doesnt do it in free rev. The spark plugs are new, plug wires are close to new, but was planning to ohm them with a dmm. The hei has a new ignition pickup, and the module along with the coil are new. The firing order is correct of course also. It is smooth at all other rpms, and i have checked every ground on the wiring harness. I have verified tdc on the balancer tab and checked the intial timing multiple times. Any ideas? Thanks
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Re: How to diagnose a random misfire?

Postby Lockwire » Tue Mar 19, 2013 8:11 pm

Who made the spark plugs ?.

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Re: How to diagnose a random misfire?

Postby crazyman » Tue Mar 19, 2013 8:16 pm

Dead spot in the TPS?

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Re: How to diagnose a random misfire?

Postby BOOT » Tue Mar 19, 2013 8:17 pm

Kinda a long shot, but try unhooking the factory tach. Well figure out what mph it is doing it then unhook the tach and take it for a drive. When factory tachs go bad they sometimes cause the hei to malfunction.
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Re: How to diagnose a random misfire?

Postby c4mo » Tue Mar 19, 2013 8:30 pm

ngk ur5, tps according to data log is fine. The tach reads the same as my data log as does the speedometer, but i could try unhooking it and see. Could the tach filter be going bad?
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Re: How to diagnose a random misfire?

Postby Strange Magic » Tue Mar 19, 2013 8:48 pm

Put another set of spark plugs in and a new set of wires. I know it sounds redundant but I would bet that it fixes the issue. These things where notorious for breaking down spark plug wires and having issues that you speak of.
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Re: How to diagnose a random misfire?

Postby c4mo » Tue Mar 19, 2013 8:52 pm

I was leading towards a new set of wires and plugs. When you say these things, are you referring to hei or these cars in general?
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Re: How to diagnose a random misfire?

Postby RednGold86Z » Tue Mar 19, 2013 8:58 pm

c4mo wrote:The problem im having is with a sbc tpi in a 1985 corvette. The problem happens around 3500-3800 rpm and it will skip back and forth, otherwise the engine is fine. Also this is under load only it doesnt do it in free rev. The spark plugs are new, plug wires are close to new, but was planning to ohm them with a dmm. The hei has a new ignition pickup, and the module along with the coil are new. The firing order is correct of course also. It is smooth at all other rpms, and i have checked every ground on the wiring harness. I have verified tdc on the balancer tab and checked the intial timing multiple times. Any ideas? Thanks


If it's almost like a total cut out of power, and the ignition seems fine without load, it's probably lean misfire. If the engine is stock, you're probably running out of fuel pressure via a bad pump or clogged filter. Get a fuel pressure gauge on there for a quick test (there's a Schroeder valve on the rail for one). If that's not crapping out, your MAF might have some problems - it's pretty easy to diagnose - just look for steadily increasing (with RPM and load) MAF counts or airflow g/sec -whatever it shows for MAF on the scan tool.

Can it pull past 3800? If it can, forget the above (well, maybe not forget...), and look for a place where the harness might be bending or being pinched when the engine torques on the mounts.

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Re: How to diagnose a random misfire?

Postby BOOT » Tue Mar 19, 2013 9:01 pm

Like I said longshot, but it's free and I always start with the free and simple stuff. Did you drop a plug when you installed them?
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Re: How to diagnose a random misfire?

Postby Strange Magic » Tue Mar 19, 2013 9:03 pm

I worked for Chevrolet during those few years. The 350's had this issue of constantly breaking down the wires. I truely believe it was from excessive load from the shear weight of the vehicle and torque converter clutch, of which the torque converter clutch never released when it should have. You had to really get on the gas for it to release, and if not it would load the engine hard. Those vettes shifter fast and to soon loading the engine pretty hard. They also where set up to run on the lean side for emissions and that wasn't good for the plugs or wires either. It's been a while, but I believe at the time the wires where grey in color and you could see carbon tracks on them, like a grey powder area or traces of grey powder.

Hopefully your not mis-diagnosing an engine miss for torque converter clutch shutter.
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Re: How to diagnose a random misfire?

Postby c4mo » Tue Mar 19, 2013 9:11 pm

Redngold, i think ive seen you on thirdgen.org. Anyway the engine will pull past 3800 it just misfires there, and doesnt after. Im using dynamic ebl ecm, so of course its map based instead of maf now. I attached a notepad datalog where you can really see it at 2:37 mark if you scroll down. My ve tables are maxed out in this area also, so more fuel cant really be added.
datalogs.zip
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Re: How to diagnose a random misfire?

Postby c4mo » Tue Mar 19, 2013 9:14 pm

boot,
I didnt drop any plugs, made sure to gap them all, and ohm them with a multimeter before putting them in. Also i Have a fp i can read while driving and fuel pressure isnt dropping.
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Re: How to diagnose a random misfire?

Postby RednGold86Z » Tue Mar 19, 2013 11:07 pm

c4mo wrote:Redngold, i think ive seen you on thirdgen.org. Anyway the engine will pull past 3800 it just misfires there, and doesnt after. Im using dynamic ebl ecm, so of course its map based instead of maf now. I attached a notepad datalog where you can really see it at 2:37 mark if you scroll down. My ve tables are maxed out in this area also, so more fuel cant really be added.
datalogs.zip


VE should almost always go up with higher load (manifold pressure), so when I see 80kPa at 100%, and 100 at 92%, I always think the VE is wrong. Also, when I see a VE dip as RPM goes up, it MUST follow a torque drop otherwise it's probably wrong. Also, check your PE Tables to see if there's something missing in that range - you'll need at least 15% enrichment in power enrichment to get to a suitable AFR.

What modifications, other than EBL and speed density have you done? If you're at 100% VE, it must be something nice. If you've used the wrong displacement and injector flowrate in the calibration, it may make your VE need to be high. If you actually need higher than 100% VE, you can usually get away with just lying with the cal, and say your engine is bigger or your injectors are smaller - either change will automatically change the pulse width.

BTW, it's me from thirdgen.org, I haven't posted on the DIY board in what is seeming like ages though - I've been in China for over 8 years now, running an EFI company and only get a few moments every few years to play with my TPI and EFI cars (I have a TPI 350 in a 79 malibu, an 89 GTA 350 TPI, a 91 Z28 that will be going back to a FIRST TPI manifold (has a carb now) and a supercharger, an 86 Z28 LG4 rust bucket, and a 69 Nova that will be going single plane EFI with a supercharger).

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Re: How to diagnose a random misfire?

Postby c4mo » Wed Mar 20, 2013 6:27 am

The mods on the car are:
355 4.030x3.48
10.2.1 compression
World product sportsman 2 heads ported
Bullet hyd roller .586/.586 226/234 at .050 112 lsa 107.5 lca
1.6 rockers
edelbrock tpi base fully ported
Tpis runners
1 5/8 longtube headers.
I'm using ford racing 30lb injectors at 42 psi, so I come up with 31.04 lb actual. I'm using a bPc
Of 138. Rbob helped with the injector compensation for the injectors. My commanded pe afr is 12.8 which is around 15% richer than the open loop commanded afr vs rpm & vac table, is this what pe table your referring to? Also, glad to hear you still have your toys to work on every once in a while.
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Re: How to diagnose a random misfire?

Postby DaveMcLain » Wed Mar 20, 2013 6:44 am

You should run it on a scope and check for bad wires, available voltage etc.


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