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  #1  
Old 08-17-2014, 09:23 PM
bkbkbkbk bkbkbkbk is offline
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Possible bad PCM?

95 4.9 5 speed, 130k miles. When it is up to temp and I stop for a minute, it will start to misfire and barely run. Throws code 181 (lean), once I get going it will run better but not perfect. Truck is typically fine when cold, but no guarantees. Seems to be worse on the hotter days.

Here are the new parts (not all replaced because of this, but some):
Distributor, coil, ICM, fuel pump, IAC.
Took it to a mechanic who spent 3 hours working with it, he monitored a lot of the performance with an OBD1 diagnostic tool, he found nothing, from talking with him he seemed fairly knowledgeable on this era of truck. Unfortubately the truck was not acting up at the time.
Could this be a bad PCM? How can I tell? What years all share the same PCM so I can look for one in a junkyard if need be?
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Old 08-18-2014, 09:39 AM
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Sounds more like an intermittant vacuum leak to me. Or, possibly an injector that isn't working quite right.

When it starts acting up, let it idle and grab a can of carb cleaner. Spray around the vacuum hose connections and intake manifold, in small shots. When (if) the engine suddenly smooths out for a few seconds, you've found where it's sucking in air.
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Old 08-18-2014, 11:00 AM
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Quote:
Originally Posted by Rogue_Wulff View Post
Sounds more like an intermittant vacuum leak to me. Or, possibly an injector that isn't working quite right.

When it starts acting up, let it idle and grab a can of carb cleaner. Spray around the vacuum hose connections and intake manifold, in small shots. When (if) the engine suddenly smooths out for a few seconds, you've found where it's sucking in air.
What would you guess the likelyhood is of PCM failure causing what is making an injector(s) not work right? When it ss acting up, I think it is only going on 3-4 cylinders, but it is hard to tell.

I have tried the carb cleaner trick (not while it is acting up) and came up short. I cant think of a plausible scenario where a truck has no vacuum leak then a huge one when it is warm. Not saying it is impossible, just looking for input as to how/what/where that could happen, and it is intermittant at that.
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Old 08-18-2014, 08:48 PM
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UPDATE
DId KOER test and got these two codes.. does this tell me anything?

225 Knock sensor not tested (ignore if not pinging) – KS
412 Idle speed system not controlling idle properly (generally idle too low) – ISC
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Old 08-18-2014, 08:54 PM
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Have you checked your o2 sensor ? They can cause this type of issue . Good one minute , bad another . Just a Thought !
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Old 08-19-2014, 05:38 AM
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How does the O2 sensor feedback to the motor and affect performance? Does it adjust air/fuel? I assumed it was a 'read - only' type sensor.
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Old 08-19-2014, 02:00 PM
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Have you checked fuel psi? I would check it with Engine Off. Then check it with Enhine running.

Check for leak down. The I6 needs 45-55 psi to run correctly. When you remove the vacuum line from the fuel psi regulator, it should increase 5psi with engine running.

The O2 sensor works in conjunction with the IAT (Intake Air Charge) sensor and the ECT(Engine Coolant Temp) sensor. The O2 sensor sends info back to the PCM, letting the PCM either widen or shorten the Injector pulse, meaning either more fuel or less. When the PCM cannot get close in either direction, based off the exhaust/O2 reading, then it usually sets a code for either Lean or Rich.
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Old 08-19-2014, 03:07 PM
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I have checked fuel pressure (not while acting up) and it has been solid in the 40s. I put in a new fuel pump 6 months ago and am assuming it is not the culprit. Are you sure it needs 45-55? I have read many places it is more like 35-40.

Does my 95 have an IAT sensor? Where is it located?

Thanks for the feedback guys, keep it coming.
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Old 08-19-2014, 09:00 PM
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Also, if it happens to be the O2 sensor, can I unplug it while it is acting up and see if it runs smoother? Will that confirm or deny it?
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Old 08-20-2014, 02:46 PM
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The IAT sensor is located in the #1 runner, passenger side. Under the Combined Air Bypass & Air diverter valve.

The I6 needs 45-50 fuel psi. I own both an I6 and 302. The 302 needs 32-35 and should increase around 5psi with the vacuum line removed and plugged from the Fuel psi regulator.

If you want, just remove the PCM and snap a few pics of the control board and let us have a look.

Also, when you throw a code for the IAC, it can be as simple as need to be cleaned, out of spec, or vacuum leak. Do you have the metal "coffee" can vacuum reservoirs? One will be on the passenger wheel well and another under the coolant/washer fluid reservoir tank. They rot under the bottom and make it difficult to see any problem. If you have a propane tank, then you can take a clear polyethylene tube, wide enough to fit in the opening and then gently crack the valve open and use the other end of the tube put near the intake manifold, the vacuum lines, the vacuum canisters. You can also use carn cleaner.
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Old 08-20-2014, 10:10 PM
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The IAC has been replaced. Can I do anything with the IAT or O2 sensor to check them? Will unplugging either while it is acting up have an affect on my problem, or will the truck not run anyway?

I have the coffee can on the passenger side, it is good. I did not know there was another under the coolant reservoir, i will check there. I will also pull the PCM and take pictures but will be a few days.
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Old 08-24-2014, 03:10 PM
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Replaced O2 sensor - no change.

Pulled PCM, visually inspected and it looks fine in my opinion. I did notice where some water has run down the black grommet over time and collected some silt toward the bottom.



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Old 08-25-2014, 07:30 PM
bkbkbkbk bkbkbkbk is offline
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Unplugged IAT and coolant temp sensors to see if anything would change, no change.

Please help, bumpty.
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Old 08-25-2014, 07:30 PM
 
 
 
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