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Multiple codes where to start!

CBII

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Hey

Not new to this at all but I have like 5 codes being spit out at me and I'm like wtf...which one do I replace.

Anyways I have a 91 WITH a BAP/MAP sensor.

I'm getting codes for the BAP, MAS, IAC, Throttle position, and manifold temp sensor.

I had a few spare MAS and swapped them out with no change.

The truck starts and runs but as soon as you start to give it gas it's like it almost dies and then takes off running! So every time you stop you have no idea if it'll stay running or die...oh and the mileage has gone to shit as well.

Oh and when I unplug the MAS or IAC it damn here quiets and runs super shitting. Plug them back in and it smooths right out.

I'm thinking it's the BAP?????:scratch:


I have no desire in dropping $600 in sensors so if anyone can help that would be great.
 
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Tbars4

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CBII

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Tbars4

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..I would try disconnecting the battery to clear the codes first..See which one pops up first..

..IMO, It is odd that you would get 3 separate sensors reading faults..My first thought is a bad ground for the computer, or battery cables..:biggthump

Edit: or a wire short now that I think about it..Have you checked the wires to all 3 of those sensors to see if they are grounding out somehow?

Possibly the wires to only one of those sensors is shorting but it's affecting all 3 sensors commonly when the computer reads it..
 
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MrShorty

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When you have multiple codes, it can be useful to identify which codes are KOEO and which are CM and which are both. SOP with EEC-IV is to resolve KOEO codes first, since they are almost always electrical. Once you are getting a pass code from the KOEO test, then move on the any KOER codes that remain. After resolving the KOER codes, then clear CM and resolve any CM codes that come back.
 
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CBII

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those codes all popped in the KOEO mode.

I replace EVERY ground strap on the engine with 4 gauge wire.

I'll double check all the plugs again for the 10th time and trace the wires back to the computer again.

Like I said I'm not new to this just frustrated with doing the little things and checking and rechecking everything.

As for the Battery....it's new and I've left the battery cable for half a day and the codes popped up right away after running for a few minutes.

I priced out a new BAP today and was told it was $167 :eek::eek::eek::eek:
 
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MrShorty

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those codes all popped in the KOEO mode.
Before the separator pulse (KOEO codes) or after the separator pulse (CM codes) or both?

Assuming they are all KOEO codes, here are a few suggestions:

1) I might start with the simplest sensor to test. IAT is a simple thermistor and is easy to test with an ohmmeter. Measure the resistance at any temperature (on a cold engine is easy because IAT should read about ambient temperature) and compare to a chart (Chilton's has the chart or I've seen it around the net in a few places). If the resistance is reasonable for the temperature the IAT is probably good, though you could test it further by heating/cooling it to see if the resistance changes as expected.

TPS is 2nd in difficulty. TPS is a simple potentiometer. You would test to make sure the main resistor has continuity, then check the "wiper" leg (I think that is what it is called) to see that its resistance changes smoothly from low to high.

BARO sensor is fairly easy to test if your multimeter has a frequency setting. Hook up your frequency meter to the BARO sensor and see if you get a reasonable signal. Attach a vacuum pump to the BARO sensor and see if the frequency changes with change in pressure.

2) 3 of the offending sensors are "powered" by the Vref circuit. It would probably be good to get a wiring diagram and a voltmeter and check Vref to make sure all these sensors are getting 5 V from the PCM. There might be a break somewhere in the Vref circuit that is preventing these sensors from getting that signal.

3) Don't neglect the possibility of a bad PCM. By the book, a bad PCM is diagnosed by process of elimination -- when all other possible causes are eliminated, then the PCM must be bad. Some have reported opening their PCM up and easily locating (sometimes fixing if they are good with soldering on printed circuit boards) burns or breaks in solder connections. Guess and check is probably the most common method of testing the PCM.
 
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CBII

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Thanks for the info. Hoping to finally source out the problem sooner or later.


Also would a failing alt cause these problems? Just a thought.
 
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MrShorty

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As the entire system is electronically based, it would seem within the realm of possibility for the main electrical source to create any number of problems for the system. If you otherwise know that the alternator is bad, I would be tempted to replace it before worrying about the individual codes. If there is no other indication that the alternator is bad, I don't know that I'd go very far pursuing this line of thinking.

In the same way, a bad ground can cause multiple issues.
 
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