96 Jimmy rough idle but no codes
#1
03-31-2011, 02:11 PM
96 Jimmy rough idle but no codes
My 96 Jimmy misses at idle and is sluggish under acceleration. No codes are being set. I have a new scan tool and hope to use it to diagnose this problem.
1. No arcing on wires, distributor, ignition module (tested with sprayed water on a dark night)
2. I've checked all the vacuum lines and PVC valve. No indication of an air leak.
Scan tool information follows:
3. The post-cat oxygen sensor has some long periods where it stays rich or lean for an extended period while other sensors are cycling more normally. Short term fuel trim B3 goes from -91.4% to 91.4%
4. The other two O2 sensors seem to move more or less in tandem, however Bank 2 seems to be "more lazy" than bank 1. The peaks and valleys are not as high and are sometimes missed all together. There are periods of time where these two sensors do not have a normal sine wave pattern and become more jagged. Short term fuel trim is between -12.5 & 10.9 for bank 1 and -13.3 & 9.4 for bank 2.
5. LT fuel trim is between -3.9 & 16.4 for bank 1 and -5.5 & 14.8 for bank 2.
6. STFT for sensor 3 is between -100 & 98.4
Is the post cat sensor used for fuel control on this vehicle?
1. No arcing on wires, distributor, ignition module (tested with sprayed water on a dark night)
2. I've checked all the vacuum lines and PVC valve. No indication of an air leak.
Scan tool information follows:
3. The post-cat oxygen sensor has some long periods where it stays rich or lean for an extended period while other sensors are cycling more normally. Short term fuel trim B3 goes from -91.4% to 91.4%
4. The other two O2 sensors seem to move more or less in tandem, however Bank 2 seems to be "more lazy" than bank 1. The peaks and valleys are not as high and are sometimes missed all together. There are periods of time where these two sensors do not have a normal sine wave pattern and become more jagged. Short term fuel trim is between -12.5 & 10.9 for bank 1 and -13.3 & 9.4 for bank 2.
5. LT fuel trim is between -3.9 & 16.4 for bank 1 and -5.5 & 14.8 for bank 2.
6. STFT for sensor 3 is between -100 & 98.4
Is the post cat sensor used for fuel control on this vehicle?
#2
03-31-2011, 02:32 PM
The post cat sensor should basically always read lean since the catalytic converter is burning up all of the extra fuel & air.
There shouldn't be that much variation in LTFT, but I guess that would be qualified by the time frame where that range was observed.
STFT for sensor 3 is erroneous and doesn't have any bearing on the operation of the engine operation. I'm surprised it even shows up in your scan tool.
Your truck should have 4 O2 sensors. B1S1 & B2S1 are the only two that are used for engine management. The other two (B1S2 & B1S3) are only used for emissions monitoring and do not get used for fuel control.
Now. Have you run any injector cleaner through the engine? Cleaned the MAF/EGR/throttle body? How do the plugs look?
There shouldn't be that much variation in LTFT, but I guess that would be qualified by the time frame where that range was observed.
STFT for sensor 3 is erroneous and doesn't have any bearing on the operation of the engine operation. I'm surprised it even shows up in your scan tool.
Your truck should have 4 O2 sensors. B1S1 & B2S1 are the only two that are used for engine management. The other two (B1S2 & B1S3) are only used for emissions monitoring and do not get used for fuel control.
Now. Have you run any injector cleaner through the engine? Cleaned the MAF/EGR/throttle body? How do the plugs look?
#3
03-31-2011, 05:24 PM
The MAF was replaced about 500 miles ago with a known good unit. I had a DTC set by the old MAF. MAF and MAP are providing consistent results using the scan tool.
I have not yet run injector cleaner through the engine. I plan on doing that and also an application of Seafoam via a vacuum hose this weekend.
I have not yet cleaned EGR or throttle body. I just purchased the new scan tool and thought I might be able to detect a lean condition if there was a problem in the EGR system. I'll clean them this weekend.
I just hate pulling plugs on these engines! I'll pull and read them tomorrow and probably put in new plugs at that time. I purchased the car in December and do not know when the plugs were last changed. I'll probably replace the distributor cap at the same time.
I was wondering if a post cat sensor was reading rich so much if that might indicate a bad cat and possibly a partially blocked exhaust. I'll have to see which of the O2 sensor was alternately reading high and low. I thought it was the post cat when setting up the tool. No reading is available for a 4th sensor.
by the way I said the MAP and MAF are giving similar readings. That is correct - the values are between 4.15 & 99.46 gram/sec from the MAF and 4.28 & 73.29 from the MAP. Are these readings normal or low - normal driving conditions with a couple of stoplights along the way. No heavy acceleration runs.
I have not yet run injector cleaner through the engine. I plan on doing that and also an application of Seafoam via a vacuum hose this weekend.
I have not yet cleaned EGR or throttle body. I just purchased the new scan tool and thought I might be able to detect a lean condition if there was a problem in the EGR system. I'll clean them this weekend.
I just hate pulling plugs on these engines! I'll pull and read them tomorrow and probably put in new plugs at that time. I purchased the car in December and do not know when the plugs were last changed. I'll probably replace the distributor cap at the same time.
I was wondering if a post cat sensor was reading rich so much if that might indicate a bad cat and possibly a partially blocked exhaust. I'll have to see which of the O2 sensor was alternately reading high and low. I thought it was the post cat when setting up the tool. No reading is available for a 4th sensor.
by the way I said the MAP and MAF are giving similar readings. That is correct - the values are between 4.15 & 99.46 gram/sec from the MAF and 4.28 & 73.29 from the MAP. Are these readings normal or low - normal driving conditions with a couple of stoplights along the way. No heavy acceleration runs.
Last edited by swartlkk; 04-07-2011 at 08:10 PM. Reason: *Combining Consecutive Posts* - Please use the edit function to add additional information in your post if another member has yet to respond.
#4
03-31-2011, 08:29 PM
My scans from my old '00 Bravada show anywhere from 4.5 to 114 g/sec (max at 4970RPM & 100% throttle angle) for MAF values. MAP sensor values from 5.3 to 28.6 inHg (13.5 to 73.4 cmHg or 17.9 to 98 kpa). What units is your MAP sensor value in?
Low MAP value = high manifold vacuum
The B1S2 is the pre-cat sensor and should show a similar trend to what is seen on the B1S1 & B2S1 sensors. B1S3 should be nearly a flat line showing rich (no oxygen).
Low MAP value = high manifold vacuum
The B1S2 is the pre-cat sensor and should show a similar trend to what is seen on the B1S1 & B2S1 sensors. B1S3 should be nearly a flat line showing rich (no oxygen).
#5
04-01-2011, 07:01 AM
[unfortunately my laptop battery died and I lost the log file for the data run whose values were quoted in earlier posts. Values in this post are from a shorter duration run the previous day]
The MAP values are 21.0 to 95.2 kPa and 6.2 to 28.1 inHg.
The B1S3 O2 sensor is definitely cycling between 0 and .780 volts. It is not a straight line but is in a lean condition most of the time (<.6v).
The MAP values are 21.0 to 95.2 kPa and 6.2 to 28.1 inHg.
The B1S3 O2 sensor is definitely cycling between 0 and .780 volts. It is not a straight line but is in a lean condition most of the time (<.6v).
#6
04-02-2011, 08:18 AM
running the injector cleaner and the seafoam now. Cleaning of MAF, EGR and throttle body on the agenda too.
Looking at the scan tool data again I see that the ignition timing advance ranges from -42.5 to +10.0. That's right a negative advance.
Thinking this may just be the scan tool I connected it to my 98 blazer. timing on the 98 is -1.5 to +40.5. More what I expected.
negative timming advance, loss of power on accelleration, high LT fuel trim? maybe a bad knock sensor?
injector cleaner, seafoam, a new distributor cap and rotor got me most of the way there, but I was still seeing some high fuel trims. Cleaned the EGR valve and picked up some improvement. Cleaned the throttle body and much better. LT fuel trim is pretty good -4.7 to 6.2 on B1 and -7 to +7 on B2. ST fuel trims are -10.2/-1-.4 to +9.4/9.4
I've decided that the 96 provides ignition advance as negative numbers and the 98 as positive numbers.
Looking at the scan tool data again I see that the ignition timing advance ranges from -42.5 to +10.0. That's right a negative advance.
Thinking this may just be the scan tool I connected it to my 98 blazer. timing on the 98 is -1.5 to +40.5. More what I expected.
negative timming advance, loss of power on accelleration, high LT fuel trim? maybe a bad knock sensor?
injector cleaner, seafoam, a new distributor cap and rotor got me most of the way there, but I was still seeing some high fuel trims. Cleaned the EGR valve and picked up some improvement. Cleaned the throttle body and much better. LT fuel trim is pretty good -4.7 to 6.2 on B1 and -7 to +7 on B2. ST fuel trims are -10.2/-1-.4 to +9.4/9.4
I've decided that the 96 provides ignition advance as negative numbers and the 98 as positive numbers.
Last edited by swartlkk; 04-07-2011 at 08:11 PM. Reason: *Combining Consecutive Posts* - Please use the edit function to add additional information in your post if another member has yet to respond.
#8
04-03-2011, 08:08 AM
The six runs that I performed to test the effect of changes ranged from a few minutes in the garage to about ten miles on the road. Bank1-LTFT has moved from -3.9/16.4 to -4.7/6.2; Bank2-LTFT from -5.5/14.8 to -7.8/7.0.
Max low LTFT+STFT is about -12 and occurs after a TOS spike followed by an RPM drop.
Probably still have some LT learning to do before I see the full effect of the changes.
BTW - when cleaning the throttle body I noticed a small wash spot on the drivers side between the front and rear injectors. It does not look like an injector leak. My fuel pressure readings indicate a leak down problem:
Crank 60 PSI
Run 56
Key-off 52
after 5-min 47
after 10-min 42
Max low LTFT+STFT is about -12 and occurs after a TOS spike followed by an RPM drop.
Probably still have some LT learning to do before I see the full effect of the changes.
BTW - when cleaning the throttle body I noticed a small wash spot on the drivers side between the front and rear injectors. It does not look like an injector leak. My fuel pressure readings indicate a leak down problem:
Crank 60 PSI
Run 56
Key-off 52
after 5-min 47
after 10-min 42
#9
04-06-2011, 06:29 AM
problem is still not solved. I have an unsteady idle. On a very short warm engine run the RPM bumped from 579 to 612. Oddly the TPS is varing from 0% to .4% without touching the pedal. I swapped a known good TPS sensor and same results. Intake manifold pressure varies from 35Kpa to 36Kpa.
I'm guessing a random misfire or weak spark at one of the cylinders. I'm planning on replacing the plugs and doing a compression check at the same time. Anything else I should look for while I am under the hood?
I'm guessing a random misfire or weak spark at one of the cylinders. I'm planning on replacing the plugs and doing a compression check at the same time. Anything else I should look for while I am under the hood?
