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Old 12-13-2008, 09:05 AM
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false OBD 2 misfire reading???

2001 chevy express 1500 4.3L auto

I'm trying to pinpoint the cause of a misfire reading. It seems like a false reading.The computer is reading severe misfire, but the van runs PERFECT.
I get almost 20 mpg's. There is no elevated HC's. No one has been able to
pinpoint the problem, including the chevy dealer. The CKP sensor was replaced. Then I switched CKP sensors from my s-10 to the van. No help-same problem. I have good compression, and the fuel system seem to be running correctly.

I had a revelation last night. The crank sensor did go out last year, causing
severe misfiring and detonation. I had to drive the van some miles in that state. I am now wondering if the catalytic converter melted causing some
exhaust restriction. The strange misfire reading showed up immediately
after the sensor was replaced and has persisted over 30,000 miles and many
tune ups. All while the van was running fine.

Can an exhaust restriction manifest as a misfire reading due to slowing the
crankshaft between powerstrokes? It seems like this could cause an uneven sine wave signal to the comp resulting in a misfire code.

I can't get my 2009 registration until I fix the problem that no mechanic
has been able to fix. In Atlanta I can't sell the van, legally drive the
van, or register it. aaaggghhh!!!

I will be posting this question in the electrical and engine forums.

Thanks

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Old 12-13-2008, 09:55 AM
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Can't answer your question but have seen something similar on my 2000 Ford Taurus. Mine always shows high mis-fire count but never sets the Check Engine Light or any codes and runs great. Does yours set codes?

I've just assumed it was a fluke of the scanner I use (www.obd-2.com). I'll be watching to see if someone smarter than me posts info.

Paul
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Old 12-13-2008, 12:57 PM
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What are the codes?
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Old 12-13-2008, 04:02 PM
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P0304 pending and mature
sometimes p0300
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Old 12-13-2008, 04:20 PM
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Do either of you have a long crank time to start after shutting it off hot?

Brian, What type of fuel injection does yours have?

Paul, clean your mass air flow sensor.
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Old 12-13-2008, 04:26 PM
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The van always starts right up.
The fuel system is the central sequential fuel injection - six poppet injectors.
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Old 12-13-2008, 04:28 PM
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I have some mass air flow cleaner, i'll try that tommorrow.
How might that be related to the problem?

Thanks for the input,
Brian
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Old 12-13-2008, 04:35 PM
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Quote:
Originally Posted by briansansone
I have some mass air flow cleaner, i'll try that tommorrow.
How might that be related to the problem?

Thanks for the input,
Brian
A dirty, or oil film from a aftermarket air filter like K&N can cause it to send the wrong signal to the ecm, saying I am reading to "thick of air"(heavy), and the ecm will lean out the fuel injection, causing the P0300 (random miss fire).

A code of P0304 is a cylinder miss fire in cylinder #4.
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Old 12-13-2008, 05:01 PM
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The problem has been p0304 for over a year. the p0300 showed up recently.
Hopefully that's not indicative of severe engine wear. Again, the engine runs
great. A little bit loud on the top end, but it never smokes, skips, knocks,
stalls, etc...

Sometimes during warm up the exhaust smells lean. Shouldn't it be rich
during warm up...??

Thanks again for the input, i'll check and clean the sensor in the morning
and post back.

Brian
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Old 12-13-2008, 05:40 PM
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I don't remember if a 2001 4.3L has the same timing sequence as a 5.0, or 5.7 Liter V8.

On the V8's you need to have a scan tool capable of setting the Cam and Crank sensors in sync. I'll have to do some research on this.

Other things come to mind. P0304 cylinder #4, check the spark plug boot, plug, and compression. A miss fire will cause the exhaust on that bank to read rich.

This brings up a thought of 'dirty', or carbon-ed up HO2 sensor. This will also cause the ECU to lean that bank out. Not likely though, because of only the cylinder #4 miss fire code.
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Old 12-13-2008, 09:41 PM
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Did your GM shop run a top end cleaner through the motor?

The 96+ 4.3 SFI motors are notorious for dirty and sticky injectors. The 4.3 that used to be in my S10 used to do it all the time, (I think it was the ethanol in the 87 gas being if I stepped up to 89 for awhile it went away). Anyway here is the bulletin

Fuel System - MIL ON/Rough Idle on Start-Up

File In Section: 06 - Engine/Propulsion System

Bulletin No.: 00-06-04-003B

Date: February, 2003

TECHNICAL
Subject:
Rough Idle After Start, And/Or A Service Engine Soon (SES) Light
(Unstick And Clean Central Sequential Fuel Injection (CSFI) Poppet
Valves or Convert to MFI)

Models:
1995-2002 Chevrolet and GMC S/T Models
1996-2002 Chevrolet and GMC C/K, M/L, G, P Models
1996-2001 Oldsmobile Bravada
1999-2000 Cadillac Escalade
with 4.3L, 5.0L or 5.7L Engine (VINS W, X, M, R - RPOs L35, LF6, L30, L31)
Except 2002 VIN X - RPO LU3 Models

This bulletin is being revised to update the Warranty Information. Please discard previous copies of Corporate Bulletin Number 00-06-04-003B with warranty chart (Section 06 - Engine). This copy is for Dealers in the state of CALIFORNIA ONLY.

Condition

Some customers may comment on rough idle after start-up, especially if the vehicle has sat overnight. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire or history misfire codes may be detected with the Tech 2 scan tool.

Cause

A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition.

Correction

A new injector unsticking and cleaning process has proven to be effective in restoring poppet valves to an "as new" condition. CSFI injector replacement should NOT be considered as a correction for this customer concern.

Check the vehicle history to determine if the vehicle is returning for a second fuel system repair for this condition. If the fuel system has been repaired for this condition previously, then inform the customer that an alternate fix is available. A new MFI fuel system has been developed that will back service the CSFI fuel system. The MFI fuel system eliminates the CSFI poppet valve ball and seat. The CSFI fuel system will need to be replaced as a unit. Refer to Central SFI to MFI Conversion instructions in this bulletin.
Notice : Individual CSFI injectors can not be replaced with MFI injectors. The entire CSFI fuel meter body will need to be replaced. The bracket used to retain the injectors in the fuel meter body is different between the CSFI and MFI unit. The unit may not seal if you mix injectors. Severe engine damage could result.

Cleaning the CSFI injectors is the preferred repair. The CSFI unit should not be replaced until cleaning has been attempted. If the vehicle should return for the same repair, then the CSFI unit can be replaced with an MFI unit. If the vehicle is out of warranty, the customer should be given the option of which fix to pursue.
Important : "GM of Canada" dealers require District Service Manager approval prior to replacing the CSFI unit with a MFI unit under warranty.

The first step of this process is to use the J 41413 Evap Pressure/Purge Station, in conjunction with the J 44466-10 pressure regulator/hose assembly, to provide the required high pressure (150 psi) source to perform the unsticking procedure. The J 39021 fuel injector tester and accessories provide the means to "energize" the injector. Some later model-year vehicles may utilize the Tech 2(R) to "energize" the injectors.

The second step of this process is to run the engine on a solution of 10% Top Engine Cleaner and 90% gasoline. This will effectively clean any deposits from the ball and seats of the poppets. It is very important that the engine fuel system is separated from the vehicle fuel system. Top Engine Cleaner may have detrimental effects on the fuel pump.
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Old 12-16-2008, 07:16 PM
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I might pull and clean the injectors just for preventative maintenance,
but the van never idles rough. It runs perfectly and absolutely fine.
I get 20 miles per gallon on the highway, and even in super hot Atlanta and
on below freezing days it starts and runs great.

I have not yet had a chance to investigate any further, or try any of
the suggestions offered here-I have been working morning to night,
using the van no less. I probably put an average of at least a hundred miles
a day on the van, and again have never had a problem in the 40K
miles I've put on the van since the crank sensor failure.
Thanks everyone,
I'll post back as soon as I can
Brian
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Old 12-16-2008, 07:19 PM
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Is there a place on the net, or resource where I can track down service
bulletins? They have been incredibly helpfull to me in the past.

Thanks again
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Old 12-27-2008, 04:07 AM
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you can get all the TSBs from the dealer.
you can also go to,
http://alldata.com/

a check of the current misfire counts with the motor running would be in order. it doesn't take much of a misfire to make the PCM unhappy.
a compression test would be a good ideal, 15 PSI lower compression on #4 can be enough to set a misfire code, but not enough for you to feel.
you said its a little bit loud on the top end, maybe a loose rocker arm?

the poppet injectors are troublesome at best. pulling the injectors to clean them won't do any good unless you send the rail out for service, and i wouldn't recommend wasting the time or money to do it.
the updated fuel rail for a V-6 isn't too expensive, around $300.00 or so for the complete assembly.
if you go with the updated fuel rail, i would go a head and replace the lower intake gaskets at the same time. also make certain you put the injectors in the right positions, the retainers on the updated rail are nearly impossible to get back out once they are locked into place.
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Old 12-31-2008, 08:55 PM
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Maybe I missed it, but has no one suggested changing the spark plugs?
I've been plagued with misfires this past year after changing the wife's Explorer's plugs. This was misfiring NEW plugs, so it was very hard to track down. I won't mention the plug brand.

.....yes I will, they were Autolites, made in Mexico.
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