Code thrown but not for my year/MAP issue

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Just now really trying to get this thing on the road. Drove it a mile down the road to get some gas. I pulled out in traffic kind of quick and it squawked and bogged for a couple of seconds and then came back around just fine but lit up the check engine light. Took it back to the house.

The codes I got were a 15 and a 34. The 15 is my CTS circuit but my book says a 34 is for a 1984- 1986 (differential pressure sensor) or for a 1987 (MAP sensor). So since mine's in 91 how does that apply?

My coolant temp wires recently came disconnected and I put them back together so I went to check those and they seem good. The ground is on a pretty rusty bolt on the intake so I cleaned that up and reattached it. While reattaching it I noticed that one of my wires that I connected from the CTS was disconnected. After cutting back the heat shrink I found that I didn't pinch down the connection on that side for whatever reason so I thought bam there's the fix. But no it wasn't.

Took it for a spin and everything seemed fine until I got on it kind of hard and it shifted. The check engine light came on and when I got home I tested it again and it still said coolant temp sensor circuit. So while I fiddle with that can somebody help me out with the 34?
 

gmbellew

Full Access Member
Joined
May 27, 2018
Posts
1,046
Reaction score
1,023
Location
Kansas city
First Name
glen
Truck Year
1990
Truck Model
suburban 1500
Engine Size
350
this ought to help
 

Attachments

  • Screenshot_20231125_192424_Adobe Acrobat.jpg
    Screenshot_20231125_192424_Adobe Acrobat.jpg
    78.7 KB · Views: 34
  • Screenshot_20231125_192414_Adobe Acrobat.jpg
    Screenshot_20231125_192414_Adobe Acrobat.jpg
    67.3 KB · Views: 34
  • Screenshot_20231125_192453_Adobe Acrobat.jpg
    Screenshot_20231125_192453_Adobe Acrobat.jpg
    93.2 KB · Views: 29
  • Screenshot_20231125_192459_Adobe Acrobat.jpg
    Screenshot_20231125_192459_Adobe Acrobat.jpg
    60 KB · Views: 32

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
29,088
Reaction score
23,900
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Both codes apply to your year.

Always deal with the lower numbered code first. So in your case, work on the CTS circuit first and get that taken care of. The code 34 may have been a false code. Also, if the harness and plug pass a visual inspection, the temp sensor itself is typically the culprit.
 

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Both codes apply to your year.

Always deal with the lower numbered code first. So in your case, work on the CTS circuit first and get that taken care of. The code 34 may have been a false code. Also, if the harness and plug pass a visual inspection, the temp sensor itself is typically the culprit.

Thanks. I didn't question the 15, it was the 34 Im confused about.
You must be registered for see images attach

Here's where some of my confusion started. I'm realizing now that they're just stopping at 87 even though the Suburbans and the Blazers go to 91. Probably should have read between the lines a little bit more, doh!

The CTS is newish and one of my previous problems so I'll go through that thing again first then. The MAP has got maybe an hour of run time since new and the vacuum lines going to it were tested with my mighty vac and held fine.

Off to test and poke around, thanks for the direction
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
CODE 15 Sets if the sensor, wires, or connections
Coolant Sensor (CTS) are open for 3 seconds.
LOW Temperature indication

CODE 34 Low or no output from sensor with engine running.
Manifold Absolute Pressure (MAP)
HIGH Vacuum


=======================================================================================


CODE 15 - COOLANT TEMPERATURE SENSOR CIRCUIT

CIRCUIT DESCRIPTION:

The coolant temperature sensor is a thermistor that controls the signal voltage to the ECM. The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore the ECM will see high signal voltage.

As the engine warms, the sensor resistance becomes less and the voltage drops. At normal engine operating temperature (85~C to 95~C), the voltage will measure about 1.5 to 2.0 volts.


TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1.Code 15 will set if:

^Signal voltage indicates a coolant temperature less than -44~C (-47~F) for 3 seconds.

2.This test simulates a Code 14. If the ECM recognizes the low signal voltage, (high temp.) and the "Scan" reads 130~C or above, the ECM and wiring are OK.

3.This test will determine if CKT 410 is open. There should be 5 volts present at sensor connector if measured with a DVOM.


DIAGNOSTIC AIDS:

A "Scan" tool reads engine temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90~C then stabilize when thermostat opens.

A faulty connection, or an open in CKT 410 or 452 will results in a Code 15.
You must be registered for see images attach

The "Temperature To Resistance Value" scale above may be used to test the coolant sensor at various temperature levels to evaluate the possibility of a "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.
You must be registered for see images attach



========================================================================================

Code 34 Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage - High Vacuum)

Circuit Description


The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will carry from about 1-1.5 volts at idle to 4-4.6 volts at Wide Open Throttle (WOT). A scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage while a high pressure (low vacuum) reads a high voltage. If the MAP sensor fails the ECM will substitute a fixed MAP value and use the Throttle Position Sensor (TPS) to control fuel delivery.

Test Description

Numbers below refer to circled numbers on the diagnostic chart.

1.This step determines if code 34 is the result of a hard failure or an intermittent condition. A code 34 will set when:
-Engine is less than 1200 rpm.
-Manifold pressure reading is less than 14 kPa, conditions met for 1 second.
OR
-Engine speed is greater than 1200 rpm.
-Throttle angle over 21%.
-Manifold pressure less than 14 kPa, conditions met for 1 second.
2.Jumping harness terminals B to C (5 volts to signal circuit) will determine if the sensor is at fault or if there is a problem with the ECM or wiring.
3.The scan tool may not display 5 volts. The important thing is that the ECM recognized the voltage as more than 4 volts indicating that the ECM and CKT 432 are OK.

Diagnostic Aids

An intermittent open in CKT 432 or CKT 416 will result in a code 34.

With the ignition ON and the engine OFF the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude.

Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a suspect sensor. Reading should be the same plus or minus .4 volt. Also, MAP output check can be used to test the MAP sensor.

-Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than .1 volt indicates a bad connector or connection. If OK, replace sensor.
You must be registered for see images attach

Note: Make sure electrical connector remains securely fastened.
You must be registered for see images attach

 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
Thanks. I didn't question the 15, it was the 34 Im confused about.
You must be registered for see images attach

Here's where some of my confusion started. I'm realizing now that they're just stopping at 87 even though the Suburbans and the Blazers go to 91. Probably should have read between the lines a little bit more, doh!

The CTS is newish and one of my previous problems so I'll go through that thing again first then. The MAP has got maybe an hour of run time since new and the vacuum lines going to it were tested with my mighty vac and held fine.

Off to test and poke around, thanks for the direction
A differential pressure sensor is what would be found on a carbureted engine while a MAP sensor is found on the EFI trucks. It doesnt matter the difference between them, the point is they are for determining the altitude (because altitude affects AFR) because thats how CCC and speed-density EFI determine the proper fueling along with a throttle position sensor.

OP you say your sensors are new. when did you last remove battery power to your truck? The codes take over 50 ignitions to clear IIRC when fixed but you dont remove battery power.

if you have replaced your CTS a long time ago its safe to say you need to either replace it (cheap auto store brand by chance?) or your wiring is compromised. a CTS can be checked with a volt meter easily.
 

SirRobyn0

Full Access Member
Joined
Dec 6, 2019
Posts
6,738
Reaction score
11,294
Location
In the woods in Western Washington
First Name
Rob
Truck Year
1984
Truck Model
C20
Engine Size
305
@Jawzjeep I don't really have much to add to the conversation, I just came along read and thought I might be able to help but Andy and Taylor have given excellent advice and there is nothing for me to add other than to say listen to their advice it's good info!
 

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
29,088
Reaction score
23,900
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
A differential pressure sensor is what would be found on a carbureted engine while a MAP sensor is found on the EFI trucks. It doesnt matter the difference between them, the point is they are for determining the altitude (because altitude affects AFR) because thats how CCC and speed-density EFI determine the proper fueling along with a throttle position sensor.

OP you say your sensors are new. when did you last remove battery power to your truck? The codes take over 50 ignitions to clear IIRC when fixed but you dont remove battery power.

if you have replaced your CTS a long time ago its safe to say you need to either replace it (cheap auto store brand by chance?) or your wiring is compromised. a CTS can be checked with a volt meter easily.

I never knew that an old code would clear after 50 ignition cycles. You've only been back a few days and you are already learning me stuff :)
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
I never knew that an old code would clear after 50 ignition cycles. You've only been back a few days and you are already learning me stuff :)
On my 1990 the ALDL and computer system would do that. So I said it just more as a "should be" lol

but checking on the 1991 in question, I did find this
SYMPTOM DESCRIPTION:
This section is used to determine the most likely cause of a trouble code that is stored in the ECM. The SERVICE ENGINE SOON or CHECK ENGINE light will only be ON if the malfunction exist under the conditions listed. If the malfunction clears, the light will go out and the code will then be stored. Any codes stored will be cleared if no problem reoccurs within 50 engine starts.
 

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Last weekend was too busy for me with 2 thanksgiving's and my birthday so I didn't get much anything done. I finally got a chance to mess with it this weekend.

I checked my coolant temperature sensor first and the wiring was good and solid but the ground was questionable like I mentioned earlier. I tested the sensor and the values checked out and matched the degrees it should.

The intake bolt that the ground was attached to was rusty like I mentioned so I pulled the ground off and ran a die down the stud to clean up the threads.

Took it for a spin heavy on the skinny pedal and got no SES lights or issues. I'm calling it a win.

@ AuroraGirl- yes both the CTS and the MAP are new. CTS has a couple hours of run time and the MAP had maybe an hour of run time. The burb just sits unless I'm driving it for testing, which is driving around the neighborhood a few times. Since it does just sit in my driveway most of the time I leave the battery disconnected until I need it. I've heard enough horror stories with sensors that were made by crap brands so I only use the AC Delco or Delphi stuff, lol, I'm trying not to shoot myself in the foot that way.

thank you for the direction and suggestions everyone
 

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Took it for a spin. Warm up then 2 miles to the store. Idle in park 5 minutes waiting for wifey. Drive to different store a mile away and CEL light comes on in the parking spot. Idle dropped a hair maybe but I left it running 5 minutes then drove a mile home. Acted fine the whole way.

Code 34

Next day Mighty vac said vac line to map sensor was shot so I replaced the line. One other line on my cruise control stuff didn't pass the mighty vac test so it got replaced too.

Couple 5 miles of spirited driving and no cel. Left it idle for 15 minutes and still no more light. Little cat smell though.

Thanks again for the pointers.
 

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Yesterday took it for a 30 minute drive with some big hills and whatnot. No lights, no issues. Another 30 minutes drive later with no issues. Calling it a win.
 

Jawzjeep

Honk if parts fall off
Joined
Jun 5, 2021
Posts
442
Reaction score
822
Location
Washington
First Name
Joshua
Truck Year
1991
Truck Model
Suburban
Engine Size
350
What did I say? It's a win until its not.. Back to issues.

Went on a quick trip for some errands. 10 minutes of driving then sitting in a parking spot waiting on the wife. I'm waiting sitting at idle, foot off brake, for 3-4 minutes. Suddenly the idle drops and the engine runs rough. Check engine light comes on. I stepped on the gas a little to bring the idle back up but its still running rough. It drove home quickly nursing the idle when needed. Checked the codes and 34. Again.

I checked the connector as described by AuroraGirl and gmbellew. I don't have a scanner so I used a multi meter and a powerprobe to find my results. Backprobed the connector. Results were the same with my old map sensor I saved and my new one in the burb currently. Numbers seemed normal unless I'm missing something.

Key on, engine off-
Green- 4.8 volts
Grey- 5.0 volts

At idle
Green- 0.9 then settled at 1.0
Grey- 5.0 volts

Green- volts went up as I revved it. Didn't go to WOT but saw 3.5 ish volts on the signal.

I left the powerprobe on the signal wire and let it idle for 15 minutes. During this time the idle dropped and it ran rough but stayed at 1.0 volts the whole time.

I tried to take note during the rough idle if I could change anything. Checked and it had no check engine light. EGR wasnt opening, Pinching closed the vacuum line to the map did nothing, wiggling the spark plug wires did nothing, spraying starting fluid at the tbi base and around the intake did nothing.

I can tell from the tailpipe its missing or something. Blows smoke rings when it does. Timing light on each wire said plugs are getting fire. Gonna pull the plugs next and check their condition.

I checked vacuum at the back port and at idle it was 20. It went down as I revved it. When the engine stumbled and the idle dropped the vacuum did too for a split second but came right back to 20.

Any suggestions? Go ring its neck?
 

squaredeal91

Full Access Member
Joined
Mar 10, 2023
Posts
1,787
Reaction score
3,141
Location
Cave junction Oregon
First Name
Greg bush
Truck Year
1991 SB
Truck Model
K30
Engine Size
5.9 Cummins 12 valve
Have you checked egr valve? Just went through this on a 95 tbi 350 with a close friend. Not sure what exactly fixed it because I replaced map sensor and egr valve at the same time per his request. And the valve was damaged looking. It ran really really good after and still is a couple months later of daily driving.
Also on my 99 the coolant temp sensor plug was bad. I was able to kill engine at idle wiggling the wire. LD showed it was Goin from correct temp to -140°f lol. It was driving similar to the other issue. Amazing how many different things can cause similar symptoms.
 

gmbellew

Full Access Member
Joined
May 27, 2018
Posts
1,046
Reaction score
1,023
Location
Kansas city
First Name
glen
Truck Year
1990
Truck Model
suburban 1500
Engine Size
350
if the code 34 is intermittent, check the vacuum lines again and wiggle the harness around and see if you can get it to throw the code again. take the sensor out of the bracket and move/twist it to manipulate the harness.

you might also consider running some ac delco top end engine cleaner through it and see if that helps the rough idle and missing. also you can check the back pressure at the O2 sensor to check for clogged exhaust. although, not sure if any of that will help with code 34...
 

Forum statistics

Threads
42,087
Posts
908,639
Members
33,560
Latest member
bgeise
Top