Faulty PCM?

[92Ty#1524]

I've been battling performance issues for quite some time and never quite got it right. Detonation to stumbling idle. Recently, while observing boost on DM, I noticed that my mechanical boost gauge did not match what the PCM was seeing. For example,

19 (gauge)=15 (on DM)
15 = 12
10 = 6.8
20" vac = 17.5"
0 = .1"

My first thought (and from searching posts) was a vac leak. I removed the vac line from the rear of upper intake and connected a hand-held vac pump and applied vac. Vac held with no drop. With the port on the intake capped, I started the engine and manipulated the vac to the MAP with the pump. As before, manifold vacuum did not match between the gauge and DM at all levels.

I have the same inconsistancy with the engine temp sensor. With the engine cold and truck parked in the shade, ambient temp is 45. Ignition on, engine not running...DM says 58 degrees. The difference is even greater once the engine warms. Temp at sensor reads 178 and DM says 206.

Finally, TPS. TPS volts and % occassionally drop off while at WOT from 100 to 76%. Ive checked all that I know and have read. Nothing appears to be wrong with the sensor. I recently cycled my throttle full span (engine off) while watching DM and caught it while screwing up. TPS % stopped at 76% (approx 3.6 volts). I continued to give more throttle. Volts rose to about 4.18 at WOT but % stayed at 76. The problem vanished like a fart after I cyled the throttle back and forth a few times. Just my luck, I was only monitoring and not recording when it happened (ugh!) I've posted a question on these issues before.


http://www.syty.net/phpBB2/viewtopic.php?p=179106&highlight=#179106

My PCM is a reman 7749. Got it that way and dont know why it was ever replaced. All engine sensors have been replaced in the past 2 years with exception to TPS. This is a 3bar setup. The truck had problems when I got it and still does in spite of the upgrades. Common sense tells me that the PCM is either not getting the proper signal from the engine sensors or its not properly calculating the inputs. My guess is I have either a grounding issue, a harness issue, or a faulty PCM. Im about replace the PCM but would rather not make a $100 guess. My wife would kick my ass if it that doesnt fix it.

Any input would be greatly appreciated :-?

 

[leroy]

Re: Faulty PCM?

I've been battling performance issues for quite some time and never quite got it right. Detonation to stumbling idle. Recently, while observing boost on DM, I noticed that my mechanical boost gauge did not match what the PCM was seeing. For example,

19 (gauge)=15 (on DM)
15 = 12
10 = 6.8
20" vac = 17.5"
0 = .1"

My first thought (and from searching posts) was a vac leak. I removed the vac line from the rear of upper intake and connected a hand-held vac pump and applied vac. Vac held with no drop. With the port on the intake capped, I started the engine and manipulated the vac to the MAP with the pump. As before, manifold vacuum did not match between the gauge and DM at all levels.

I have the same inconsistancy with the engine temp sensor. With the engine cold and truck parked in the shade, ambient temp is 45. Ignition on, engine not running...DM says 58 degrees. The difference is even greater once the engine warms. Temp at sensor reads 178 and DM says 206.

Finally, TPS. TPS volts and % occassionally drop off while at WOT from 100 to 76%. Ive checked all that I know and have read. Nothing appears to be wrong with the sensor. I recently cycled my throttle full span (engine off) while watching DM and caught it while screwing up. TPS % stopped at 76% (approx 3.6 volts). I continued to give more throttle. Volts rose to about 4.18 at WOT but % stayed at 76. The problem vanished like a fart after I cyled the throttle back and forth a few times. Just my luck, I was only monitoring and not recording when it happened (ugh!) I've posted a question on these issues before.


http://www.syty.net/phpBB2/viewtopic.php?p=179106&highlight=#179106

My PCM is a reman 7749. Got it that way and dont know why it was ever replaced. All engine sensors have been replaced in the past 2 years with exception to TPS. This is a 3bar setup. The truck had problems when I got it and still does in spite of the upgrades. Common sense tells me that the PCM is either not getting the proper signal from the engine sensors or its not properly calculating the inputs. My guess is I have either a grounding issue, a harness issue, or a faulty PCM. Im about replace the PCM but would rather not make a $100 guess. My wife would kick my ass if it that doesnt fix it.

Any input would be greatly appreciated :-?

 

[leroy]

The TPS problem is symptomatic of a bad tps sensor.

Which coolant sensor are you monitoring, and how are you determining the temperature at the sensor?

Do you have datamaster set up for 3 bar?

Let me know if you need an ecm for a substitution test.

Hope this helps and sorry about the last post.....ooops.

Jim

 

[92Ty#1524]

I used a Raytek infrared thermometer at the base of and on the temp sensor on the lower intake from a 3" range.

As you suggested, I will likely replace the TPS - for good measure if nothing else. I wasnt prepared to condem it since the voltage WAS rising with increased throttle. However, to me, it seems like a common symptom of a bigger problem since the other "presumed good" sensors seem to be messing up as well. I question the PCM since its a common denominator, so to speak. Dont get the impression that I'm challenging your ideas. I'm not.

DM is setup for 3bar.

A spare would be great! I've been trying to get with a local guy (Erik) to borrow a PCM out of one of his trucks but havent been able to find the time since he drives his daily.

 

[leroy]

There is a diagram or chart in the service manual that lists expected voltages for the pins on the ecm. You may want to check those.

PM me if you still want an ecm to try.

Jim

 

[0966Sy]

Jack, I would suggest trying my Ty's ECM in your Ty before you go any further as to buying a new one. I'd be glad to swap them out when you have time off from work. I got tired of the bullshit at work and quit last week. I'll be working again when i start school March 15th. I could pull my ECM while the Ty's in the Bodyshop. they will have to wait on the cladding from the stealership.

Bad news tonight, Some lady broadsided the rear passenger of the Typhoon :evil: . She decided she didnt want to make the right turn on the shoulder and switched back into my lane :fist: slaming her buick into me while i passed her. She took out the last three cladding peices behind the door leaving the rollpan and the door cladding and skirt below untouched. Minor body work thank the lord for that.

Sorry for the thread Jacking i had to vent.