P1257

I have a '95 Riviera SC. Keeps throwing P1257 code, but ONLY when car is cold (less than 5 minute warm-up) & under 3000 RPM (2500-2600). If car is warm OR 3000 or above RPM, no code. Happens in the first few minutes after I leave home every day, starting up an incline, boost duty under 100%. Does NOT happen any other time. If I downshift manually to get RPM's up, does not happen. If I drive around the block & do it again, does not happen. Otherwise, runs great! Any ideas? I've replaced everything I can think of pertinent to the problem; MAF, EGR, PCM, EBT, Boost Solenoid & Actuator, H2OS sensor, bad vacuum lines, fuel pressure regulator, injectors, EVAP valve. Thanks.

Doesn't have the solution, nor the same issue. Read my post, please.

DTC P1257 Supercharger Engine Overboost



Circuit Description
Under most conditions, the PCM commands the boost control solenoid to operate at a 100 percent PWM to allow full intake boost pressure upon demand. However, if reverse gear is selected, the PCM detects rapid deceleration, or engine load is extremely high, reduced boost pressure is desired. Under these conditions, the PCM commands the boost control solenoid to operate at a 0 percent PWM, which opens the bypass valve to reduce boost pressure by recirculating it back through the supercharger inlet.

Conditions for Setting the DTC
•Intake air temperature greater than -10°C (14°F).
•Engine torque exceeds a maximum torque threshold value. This value varies depending on commanded gear and engine rpm.
•Above conditions for at least 6 seconds.
Action Taken When the DTC Sets
•The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
•The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.
Conditions for Clearing the MIL/DTC
•The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
•The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
•The DTC can be cleared by using the scan tool.
Boost Control System Hose Routing


(1) Vacuum Signal to Bypass Valve Actuator  
(2) Boost Signal to Fuel Pressure Regulator  
(3) Bypass Valve Actuator
(4) Boost Signal to Bypass Valve Actuator  
(5) Boost Control Solenoid
(6) Boost Source to Boost Control Solenoid  

Diagnostic Aids
Check for the following conditions:

•Boost Control Solenoid control circuit shorted to ground.
•Binding Bypass Valve. Refer to Supercharger Section 6G.
•Sticking or misadjusted Bypass Valve Actuator. Refer to Supercharger Bypass Valve Actuator.
An intermittent may be caused by rubbed through wire insulation.

Intermittent test - Disconnect PCM and install a J 39200 Digital Multimeter to monitor voltage between the boost control solenoid control circuit at the PCM harness connector and ground. With the key ON, observe voltage while manipulating related connectors and wiring harness. If the failure is induced, the voltage display will change.

Test Description
Number(s) below refer to Step number(s) in the diagnostic table.

1.Checks the Boost Control Solenoid control circuit. A short to ground in the Boost Control Solenoid control circuit will keep the bypass valve closed when the PCM is commanding it open, possibly causing an overboost condition during high engine load situations.
4.No inlet vacuum to the bypass valve actuator may cause the bypass valve to remain closed during deceleration. This condition may be perceived as a sail-on, possibly accompanied by a rough idle.
5.Checks for a sticking boost control solenoid.
13.This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.
DTC P1257 Supercharger Engine Overboost Step
Action
Value(s)
Yes
No

1
Was the Powertrain On-Board Diagnostic (OBD) System Check performed?
—
Go to Step 2
Go to the Powertrain On Board Diagnostic (OBD) System Check

2
Important: Before continuing with diagnosis, visually and physically inspect for damaged, disconnected, or incorrectly routed boost control system hoses. Refer to Boost Control System Hose Routing.

Select DTC info, Last Tst Fail and note any other DTCs set.

Are any other DTCs stored?
—
Diagnose the other DTCs first - Go to Powertrain Control Module Diagnosis
Go to Step 3

3
 1. Disconnect the Boost Control Solenoid electrical connector.
 2. Turn ON the ignition switch.
 3. Connect the test light between the Boost Control Solenoid harness connector terminals.
Is the test light ON?
—
Go to Step 9
Go to Step 4

4
 1. Disconnect the inlet vacuum signal hose from the Bypass Valve actuator. Refer to Boost Control System Hose Routing.
 2. Connect a vacuum gauge to read the inlet vacuum signal to the Bypass Valve Actuator.
 3. Start the engine and idle in Park.
 4. Observe the vacuum gauge reading.
Does the vacuum gauge indicate vacuum greater than the specified value?
12 in. Hg
Go to Step 5
Go to Step 11

5
 1. Reconnect the Boost Control Solenoid electrical connector and the inlet vacuum signal hose.
 2. Disconnect the boost signal hose between the Boost Control Solenoid and the Bypass Valve Actuator. Refer to Boost Control System Hose Routing.
 3. Connect the vacuum gauge to read the boost signal from the Boost Control Solenoid. Refer to Boost Control System Hose Routing.
 4. Turn the Boost Control Solenoid OFF using the scan tool output control function.
 5. Observe the vacuum gauge.
Does the vacuum gauge indicate vacuum greater than the specified value with the Boost Control Solenoid commanded OFF?
12 in. Hg
Go to Step 6
Go to Step 12

6
 1. Check for a restricted or damaged boost signal hose between the Boost Control Solenoid and the Bypass Valve Actuator. Refer to Boost Control System Hose Routing.
 2. If a problem is found, repair as necessary.
Was a problem found?
—
Go to Step 15
Go to Step 7

7
 1. Check for the following conditions:
◦Binding Bypass Valve.
◦Sticking or misadjusted Bypass Valve Actuator linkage.
 2. If a problem is found, repair as necessary. Refer to Supercharger.
Was a problem found?
—
Go to Step 15
Go to Step 8

8
Replace the Bypass Valve actuator. Go to Supercharger Bypass Valve Actuator.

Is action complete?
—
Go to Step 15
—

9
 1. Turn OFF the ignition switch.
 2. Disconnect the PCM.
 3. Turn ON the ignition switch.
 4. Observe the test light.
Is the test light ON?
—
Go to Step 10
Go to Step 13

10
Locate and repair short to ground in Boost Control Solenoid control circuit. Refer to Repair Procedures in Electrical Diagnosis.

Is action complete?
—
Go to Step 15
—

11
Repair restricted or damaged inlet vacuum signal hose to the Bypass Valve Actuator or blocked inlet vacuum source. Refer to Boost Control System Hose Routing.

Is action complete?
—
Go to Step 15
—

12
 1. Check for a restricted or damaged boost source hose to the Boost Control Solenoid. Refer to Boost Control System Hose Routing.
 2. If a problem is found, repair as necessary.
Was a problem found?
—
Go to Step 15
Go to Step 14

13
Replace the PCM.

Important:: The replacement PCM must be programmed. Go to Powertrain Control Module Replacement/Programming.

Is action complete?
—
Go to Step 15
—

14
Replace the Boost Control Solenoid. Go to Boost Control Solenoid.

Is action complete?
—
Go to Step 15
—

15
 1. Review and record scan tool Fail Records data.
 2. Clear DTCs.
 3. Operate the vehicle within Fail Records conditions as noted.
 4. Monitor Specific DTC info for DTC P1257 on the scan tool.
Does the scan tool indicate DTC P1257 failed this ign?
—
Go to Step 2
System OK

Thanks, Scott. However, I have already done all of the above. Everything checks out & works fine UNLESS the following 3 conditions are present:

1) Within a half mile of starting the car (car not totally warmed up).
2) Going up a moderate incline.
2) RPMs less than 3000.

If ANY of these 3 are not present, the car runs fine forever, and, after the car is warm, just repeating the trip/path does NOT set the code, no matter how many times you do it. NEVER happens at any other time. Doesn't make sense; happens only at a specific time, place, engine speed.

Any other insight will be appreciated.

Thanks,
Terry

Is the valve inside the S/C actually moving? Does it have full range of motion? Is part of the blade broke of inside the S/C?

Welcome Terry, Hope we can figure out the problem. This is by far THE BEST site for the 95-99 Riviera.

Could this code set if it isn't downshifting properly?
"•Intake air temperature greater than -10°C (14°F).
•Engine torque exceeds a maximum torque threshold value. This value varies depending on commanded gear and engine rpm. "

The symptoms are on incline and lower rpm...

Karma wrote:

Could this code set if it isn't downshifting properly?
"•Intake air temperature greater than -10°C (14°F).
•Engine torque exceeds a maximum torque threshold value. This value varies depending on commanded gear and engine rpm. "

The symptoms are on incline and lower rpm...

The PCM sees above threshold torque amount, assumes S/C is putting out too much boost. it's certainly possible. Low RPM, super high engine load uphill.....

Maybe TCC isn't disengaging at the right time?

Abaddon wrote:

Karma wrote:

Could this code set if it isn't downshifting properly?
"•Intake air temperature greater than -10°C (14°F).
•Engine torque exceeds a maximum torque threshold value. This value varies depending on commanded gear and engine rpm. "

The symptoms are on incline and lower rpm...

The PCM sees above threshold torque amount, assumes S/C is putting out too much boost. it's certainly possible. Low RPM, super high engine load uphill.....

Maybe TCC isn't disengaging at the right time?

That was my train of thought.. but TCC clutch fail should also set a code...

Scott - valve has full range of motion. I guess could be broken inside, but why would it only show up under the conditions I mentioned? Would think that would be a continuous problem.

Larry - could be, but no other codes set. And, after car is warm (for less than 5 minutes) RPM doesn't matter, even below 2500.

Thanks,
Terry

tloukaitis wrote:

Scott - valve has full range of motion.  I guess could be broken inside, but why would it only show up under the conditions I mentioned?  Would think that would be a continuous problem.  

Larry - could be, but no other codes set.  And, after car is warm (for less than 5 minutes) RPM doesn't matter, even below 2500.

Thanks,
Terry

I think monitoring boost with a gauge would be a valuable input.  Would like to know the boost when the code is set. Might have something to do with temperature's affect on resistance as far as wiring goes. Also, the PCM gets the boost value from the MAP sensor, correct? Might that be malfunctioning while cold? Just trying to think outside the box since everything else has been checked diagnostically.

Technically, the PCM doesn't even monitor certain things until the car is warmed up (closed loop). This is probably why it makes no difference of RPM when it's hot. When it's cold, it ignores some parameters. Also, a cold engine can technically "absorb" more boost with less of a load on it. That may be why the warm/cold conditions are different.

LARRY70GS wrote:

Also, the PCM gets the boost value from the MAP sensor, correct?  Might that be malfunctioning while cold?  

No MAP sensor on the 95.

Karma wrote:

LARRY70GS wrote:

Also, the PCM gets the boost value from the MAP sensor, correct?  Might that be malfunctioning while cold?  

No MAP sensor on the 95.

How does the PCM know what boost is then? It has to get the value from somewhere if this code indicates overboost.  Can sensor values be monitored real time like OBD2?

LARRY70GS wrote:

Karma wrote:

LARRY70GS wrote:

Also, the PCM gets the boost value from the MAP sensor, correct?  Might that be malfunctioning while cold?  

No MAP sensor on the 95.

How does the PCM know what boost is then?  Can sensor values be monitored real time like OBD2?

The 95 has no idea how much boost there actually is. Just the variables that means it should/shouldn't be there. Not a value you can read with a scanner like you can with the OBD2. And all its calculations use the MAF, so the trick of unplugging a suspected bad MAF and letting it run on MAP on the series 2 won't work.

O. K. Maybe this could be a solution? The car has 112,000 miles on it. I don't know that the trans. fluid has ever been changed. Running at 3000+ RPM (causing issue not to show up) downshifts it to 2nd (sometimes I do it manually to force the RPMs up), I'm sure. Also, the "Traction Control Off" light sometimes comes on when I start up & stays on for a few minutes, then goes off by itself (this can also happen when it's warm, too); it's never on when P1257 is set, though. Could it be possible changing the trans. fluid could help? Otherwise, the transmission is very smooth & firm, no problems.

Karma wrote:

LARRY70GS wrote:

Karma wrote:

LARRY70GS wrote:

Also, the PCM gets the boost value from the MAP sensor, correct?  Might that be malfunctioning while cold?  

No MAP sensor on the 95.

How does the PCM know what boost is then?  Can sensor values be monitored real time like OBD2?

The 95 has no idea how much boost there actually is. Just the variables that means it should/shouldn't be there. Not a value you can read with a scanner like you can with the OBD2.  And all its calculations use the MAF, so the trick of unplugging a suspected bad MAF and letting it run on MAP on the series 2 won't work.

OK, I can't think of anything else right now. OBD1 kinda sucks.

tloukaitis wrote:

O. K.  Maybe this could be a solution?  The car has 112,000 miles on it.  I don't know that the trans. fluid has ever been changed.  Running at 3000+ RPM (causing issue not to show up) downshifts it to 2nd (sometimes I do it manually to force the RPMs up), I'm sure.  Also, the "Traction Control Off" light sometimes comes on when I start up & stays on for a few minutes, then goes off by itself (this can also happen when it's warm, too); it's never on when P1257 is set, though.  Could it be possible changing the trans. fluid could help?  Otherwise, the transmission is very smooth & firm, no problems.

I wouldn't call it a solution. It is a thought and a stab in the dark at best. I don't think it would be a bad thing to give the trans a fluid flush and filter. But for this code I think you are best getting a boost gauge and seeing what your boost is *really* doing when it sets the code. Since this code is actually pretty specific, the system that *should* be responsible for setting it is rather small, and you say you went through the whole procedure that Scott posted.(You did right? Like step by step the whole procedure as best as you can? multi-meter, PCM disconnect and measuring, vacuum measurement at the various steps...)

Correct, I went through the whole procedure. Problem is, if it's not giving a code at the time, everything shows as good. I was just thinking it could be caused by the trans. not downshifting when it should, causing the overboost/torque. Where would I connect a boost gauge?

Thanks,
Terry

There is a thread for adding a boost gauge:
https://rivperformance.editboard.com/t189p60-write-up-installing-boost-gauge

The above page shows a pic where to connect in on the 95. Basically just tee it in on the middle connector that also goes to your fuel pressure regulator, though any of those 3 lines will also work.

Another thought... under load on an incline the engine can also be twisting a bit against the engine mounts.(which is normal) A bad bit of wire in the loom for the boost system could be loosing connection when this happens.

Here's the latest. I bought & installed a Boost Gauge; here's the results:


1) Normal driving, car cold, scenario as above, no boost present, code is thrown.

2) Normal driving, car warmed up, above scenario, 1-2 lbs. boost, no code.

3) Normal driving, car cold, manually downshift to keep RPMs close to 3000, 1-2 lbs. of boost, no code.


What should this be telling me?

Normal boost on the Series 2 is about 8 psi. I wonder if the supercharger itself is worn. Ever had it off?

I dont think thats his WOT boost. I think hes saying thats what his car is reaching in normal driving conditions? If that is the case and down shifting cures it, I would try driving under the conditions that set the code and hold light pressure on the brake to keep the Torque Converter Clutch out of lockup. Might be engaging early and putting too much load on the motor? The above diagnostic test says calculated load can effect Bypass Solenoid output. Dont know if it would set that code but cant hurt to try.

Correct, that's not WOT. WOT is around 7-8. Correction to #3:

3) Normal driving, car cold, manually downshift to keep RPMs close to 3000, 10 inches of vacuum, no boost called for, no code.

I like the "brake" idea; I'll try that next.

Thanks, Terry