North American Turbocoupe Organization



Help with diagnostic codes and finding parts
mchenpa Offline
Junior Member
#1
Although the car seems to pull well and gets decent mileage, I still have a random high idle usually corrected by restarting that can include a stalling coming down from high speed (45-50) or at very low speed (parking lot). I followed the posts for base idle and TPS adjustment, so I pulled codes.

This is what came back from a KOEO and a KOER w/WOT:
* 25®: Knock not sensed during Dynamic Response Test
* 34®: EGR not opening properly
* 42®: HEGO(HO2S) sensor voltage high / system rich
* 72© which the book says is a System power circuit fault MCCA Message Center Control Assembly circuit failure.

It’s a 28k mile 1988 5-speed with a new 255 fuel pump, new fuel filter, new tank, and a new TPS. It has a manual boost control (I did get an 81(O) code for the missing inlet actuator) and cone filter.
Finally, the questions:
1. Where can I find an EGR or Knock Sensor; is there a substitute or another option? 
2. Where is the Knock Sensor on these and is this a dry job or will I end up with coolant all over me?
3. Any thoughts on this MCCA circuit? I don't know what that is and have not been able to find anything online. It sounds bad.
4. I know it’s 36 years old and it sat a few years but are all of these really bad or is there something in common that is more likely?

I'm sorry if there is already a post - or two - on these questions and I hope I gave enough information.

Thanks in advance for the help !!
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Jeff K Offline
Administrator
#2
The code 25 is there because you did not tap on the intake manifold with a small hammer, wrench, etc during the self test to "exercise" the knock sensor. Knock sensor failure is VERY VERY rare as all it is just a piezoelectric transducer (no moving parts, etc.). It is much like the piezo phonograph cartridges from the old old days of crappy "crystal" phonograph cartridges.

I bought an EGR valve from Rock Auto a number of years ago. No clue if RA still has them or not.

Ignore the code 72. 72 is momentary electrical power interruption. I have had a CM 72 come up once or twice over the last 20+ years of TC ownership and the car has always run perfectly.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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anasazi4st Offline
Senior Member
#3
(04-22-2024, 08:21 PM)mchenpa Wrote: Although the car seems to pull well and gets decent mileage, I still have a random high idle usually corrected by restarting that can include a stalling coming down from high speed (45-50) or at very low speed (parking lot). I followed the posts for base idle and TPS adjustment, so I pulled codes.

This is what came back from a KOEO and a KOER w/WOT:
* 25®: Knock not sensed during Dynamic Response Test
* 34®: EGR not opening properly
* 42®: HEGO(HO2S) sensor voltage high / system rich
* 72© which the book says is a System power circuit fault MCCA Message Center Control Assembly circuit failure.

It’s a 28k mile 1988 5-speed with a new 255 fuel pump, new fuel filter, new tank, and a new TPS. It has a manual boost control (I did get an 81(O) code for the missing inlet actuator) and cone filter.
Finally, the questions:
1. Where can I find an EGR or Knock Sensor; is there a substitute or another option? 
2. Where is the Knock Sensor on these and is this a dry job or will I end up with coolant all over me?
3. Any thoughts on this MCCA circuit? I don't know what that is and have not been able to find anything online. It sounds bad.
4. I know it’s 36 years old and it sat a few years but are all of these really bad or is there something in common that is more likely?

I'm sorry if there is already a post - or two - on these questions and I hope I gave enough information.

Thanks in advance for the help !!

You can run without a knock sensor, I know several here over the years have said they've done it. It is a "dry" job, on the right side of the engine attached to the upper intake manifold, located below the throttle body. If you are running the correct timing, using good quality premium fuel and have the Premium Fuel switch set you likely aren't having any valve knock anyway, which is what the knock sensor is designed to suppress. When I pull codes I ignore the 25 as I know what it is (I think mine might actually even be broken). I don't think you'll be able to find a replacement unless it's from a salvage yard.
Another proud dues-paying member.

1987 Turbo Coupe w/T5OD, 8.8 axle, grey smoke; most options. Got it in 1991 with 41K miles: 3 turbos, 2 heater cores, 3 T5OD full rebuilds, 6 clutches, 1 head gasket, 2 Teves II ABS units, etc. later....
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mchenpa Offline
Junior Member
#4
(04-24-2024, 04:30 PM)Jeff K Wrote: The code 25 is there because you did not tap on the intake manifold with a small hammer, wrench, etc during the self test to "exercise" the knock sensor.  Knock sensor failure is VERY VERY rare as all it is just a piezoelectric transducer (no moving parts, etc.). It is much like the piezo phonograph cartridges from the old old days of crappy "crystal" phonograph cartridges.

I bought an EGR valve from Rock Auto a number of years ago. No clue if RA still has them or not.

Ignore the code 72. 72 is momentary electrical power interruption. I have had a CM 72 come up once or twice over the last  20+ years of TC ownership and the car has always run perfectly.

Thank you for the detail - it helps put things in context.
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mchenpa Offline
Junior Member
#5
(04-28-2024, 12:41 AM)anasazi4st Wrote:
(04-22-2024, 08:21 PM)mchenpa Wrote: Although the car seems to pull well and gets decent mileage, I still have a random high idle usually corrected by restarting that can include a stalling coming down from high speed (45-50) or at very low speed (parking lot). I followed the posts for base idle and TPS adjustment, so I pulled codes.

This is what came back from a KOEO and a KOER w/WOT:
* 25®: Knock not sensed during Dynamic Response Test
* 34®: EGR not opening properly
* 42®: HEGO(HO2S) sensor voltage high / system rich
* 72© which the book says is a System power circuit fault MCCA Message Center Control Assembly circuit failure.

It’s a 28k mile 1988 5-speed with a new 255 fuel pump, new fuel filter, new tank, and a new TPS. It has a manual boost control (I did get an 81(O) code for the missing inlet actuator) and cone filter.
Finally, the questions:
1. Where can I find an EGR or Knock Sensor; is there a substitute or another option? 
2. Where is the Knock Sensor on these and is this a dry job or will I end up with coolant all over me?
3. Any thoughts on this MCCA circuit? I don't know what that is and have not been able to find anything online. It sounds bad.
4. I know it’s 36 years old and it sat a few years but are all of these really bad or is there something in common that is more likely?

I'm sorry if there is already a post - or two - on these questions and I hope I gave enough information.

Thanks in advance for the help !!

You can run without a knock sensor, I know several here over the years have said they've done it. It is a "dry" job, on the right side of the engine attached to the upper intake manifold, located below the throttle body. If you are running the correct timing, using good quality premium fuel and have the Premium Fuel switch set you likely aren't having any valve knock anyway, which is what the knock sensor is designed to suppress. When I pull codes I ignore the 25 as I know what it is (I think mine might actually even be broken). I don't think you'll be able to find a replacement unless it's from a salvage yard.

Thanks for the help.
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mchenpa Offline
Junior Member
#6
Quick update and next set of questions:

First, sorry for taking so long to say thanks for the feedback on the codes. Your comments on the 72 and 25 make sense and while I was changing AC line seals and the dryer/accumulator I found a sticker saying that TSB93-26b was performed on this car and it might throw a code 34 so I'm not worried about that one anymore either. I still replace the dried/cracked EGR hose though.

Since I last posted I have confirmed timing at 10 degrees and I have confirmed it pulls 18 inches of vacuum at idle. Since I was right there working on the dryer/accumulator I replaced the O2 sensor (it looked fine and it looked newish). I replaced the plugs (gapped to .28-.30) and confirmed the TPS.

I followed the base idle reset and it seemed to work. I took it out and ran it pretty hard for about 30 miles and it was great but then it idled up to 2500 and would not come back down. The strange thing is that before it seemed like it would idle up if I was "getting on it" and this time it happened after 5–7-minute cruise in 5th at about 2500 rpm. Shut it off/on and it reset but the next day it was terrible again.

I disconnected the IAC and set the idle with full accessory load; it wants to be around 11-1200 to run smoothly. Even with the IAC disconnected it still idles up from 1100-1200 to 1400-1500 when it gets warm so I am baffled.

It also bogs off the line or on "tip-in" in neutral even when it's at operating temp. This car sat for years and has a new fuel tank, filter, and pump.

With no codes to act on is the next step injectors and/or regulator? Maybe temp sensor?
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Mikey97D Offline
Senior Member
#7
Although vacuum numbers look good, do you have a smoke machine to check the vacuum lines for leaks?
1988 TC, 5 spd, Stinger 3" Exhaust, Schneider Roller Cam, -4° Cam Pulley, Cone Filter, Gilles Boost Control Valve set at 17 psi, Walbro 255 lph, CHE Rear Lower and Upper Control Arms, Braided Brake Lines, Hawk HPS 5.0 Front and HPS (F) Rear, CRES Inserts in front calipers, and '93 Cobra Wheels with General 235/50R17 Tires.   
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mchenpa Offline
Junior Member
#8
(05-06-2024, 02:58 PM)Mikey97D Wrote: Although vacuum numbers look good, do you have a smoke machine to check the vacuum lines for leaks?

Great minds think alike.  I ordered the parts to make one about the time I threw this post up but I haven't put it together yet. Will see how that goes this week if time permits and if it reveals anything.
Mikey97D likes this post
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Mikey97D Offline
Senior Member
#9
(05-06-2024, 07:20 PM)mchenpa Wrote:
(05-06-2024, 02:58 PM)Mikey97D Wrote: Although vacuum numbers look good, do you have a smoke machine to check the vacuum lines for leaks?

Great minds think alike.  I ordered the parts to make one about the time I threw this post up but I haven't put it together yet. Will see how that goes this week if time permits and if it reveals anything.
My thought for checking vacuum has to do with the clutch push in and stumble, not the high idle.  

You have already gone through calibrating the IAC and from shutting the car down and having it reset, we know that isn't a vacuum problem.  The temperature sensor hole in the intake manifold area often gets plugged with crap. Clean that out and maybe that may help with the idle problem.
1988 TC, 5 spd, Stinger 3" Exhaust, Schneider Roller Cam, -4° Cam Pulley, Cone Filter, Gilles Boost Control Valve set at 17 psi, Walbro 255 lph, CHE Rear Lower and Upper Control Arms, Braided Brake Lines, Hawk HPS 5.0 Front and HPS (F) Rear, CRES Inserts in front calipers, and '93 Cobra Wheels with General 235/50R17 Tires.   
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