Help with 1996 Silverado intermittent missing at part throttle

stryped

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I currently have no code. However I took data on my scanner twice. The truck sputtered and ran a little rough as soon as I left the driveway. It seem though the further I went it started to get better. Truck has had a catylist system efficiency below threshold for a number of months. I assumed it was a catalytic converter and would not be related. Truck has 342,000 miles. New plugs, wires, cap and rotor in April. Today I replaced Ignition control module and ignition coil. Ensured continuity with battery ground on the ICM harness. Ensured continuity on the white wire from computer to ICM.

Problem started as what felt like "slipping" several weeks ago when leaving for work. Did not seem to be getting as many miles pertank either. ) Never had that problem again for the rest of the day. This lasted for a few weeks. I put two bottles of fuel injection cleaner in tank and filled up tank. A few days later went out to start and took forever to start. Once did ran rough. Got the code P1351. Did above diagostics. Getting 6 colts to the ICM harness. (calls for 1-4 in book). Checked crank sensor. It is supposed to output "approximately" 5 volts but is outputting 4. Cleaned with a wire brush chasis ground. Code has not come back yet but I have not driven truck other than to do the data test a few minutes ago. (And still seem to be exhibiting problems). All sensors have he reguired supply voltage).

On a whim I checked fuel pressure. It was within specs at 64 psi. (Calls for 60-65). However, when turning the key off the pressure drops down a great deal. In 5 minutes it dropped 30 psi. I think my Haynes manual says this is abnormal but not sure.

is the truck safe to drive to work? It seems to get better as it warms up but I dont want to get stranded. It is my daily driver and I dont know what to do.

Here is the data I downloaded:


Location #1 (Test 1)

DTC-cnt 0

Fuel Sys 1 OL

Fuel Sys 2 OL

Load_PCT(%) 4. 7

ETC (F) 108

Shrt Ft1 (%) -3. 1

LongFt1(%) 0. 0

Shrt FT2 (%) -3. 9

LongFT2 (%) 0. 0

MAP (in HG) 11. 5

RPM (/min) 668

VSS (MPH) 0

Spark ADVC -19

IAT (F) 70

MAF (1b/min) 1. 198

TP (%) 0. 0

02sB1S12—B2S12—

O2B1S2 (v) 0. 230

SHRTFTB1S1 (%) -3. 9

O2B1S2 (v) 0. 340

O2B2S1 (V) 0. 130

SHRT FT B2S1 (%) -5. 5

O2B2S2 (v) 0. 070

OBD OBD2

Location #2 (Test 2)

DTC-cnt 0

Fuel Sys 1 CL

Fuel Sys 2 CL

Load_PCT(%) 17. 6

ETC (F) 205

Shrt Ft1 (%) 1. 6

LongFt1(%) 0. 0

Shrt FT2 (%) 1. 6

LongFT2 (%) 0. 0

MAP (in HG) 26. 6

RPM (/min) 1505

VSS (MPH) 41

Spark ADVC -15

IAT (F) 70

MAF (1b/min) 8. 279

TP (%) 42. 4

02sB1S12—B2S12—

O2B1S2 (v) 0. 100

SHRTFTB1S1 (%) 3. 9

O2B1S2 (v) 0. 670

O2B2S1 (V) 0. 110

SHRT FT B2S1 (%) 3. 1

O2B2S2 (v) 0. 800

OBD OBD2
 

kev2

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getting the trouble tree for p1351 BUT ...some questions

you say feeling is SLIPPING - that suggests trans issue - can you clear that up.
is this a 5,7L or a 4.3 or?
Top data is cold engine then lower is warmed engine?
one at idle then 41mph why?
I see a 11.5 invac ? at iidle
This vehicle always have us double checking that fuel inj spider assy- FPR leaking?

the vehicle is STOCK - parts are OEM type?
 

stryped

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It is like it misses at a certain throttle setting. Only while driving and only intermittently. Bone stock. 350 auto 342,000 miles. In April ignition control module went out. Replaced it with Autozone unit. Also replaced plugs/wires/cap/rotor. Truck ran great until one month ago. Sometimes runs great, sometimes has this problem. Seems worse when enigne is very light throttle under almost no load like a flat highway.

I checked TP sensor according to Haynes manual. Checked ok. Replaced ignition coil. I also replaced fuel pump. (The sending unit went bad so I replaced the whole assembly).

Some tests were off. Wire to ignition control module suppsed to have 1-4 volts cranking. Has 6. Directions say to then check CKP sensor. The output was either one volt more or less than specified, I cant remember. (I assumed this might be within the error of the multimeter). It is confusing because it basically says to replace the CKP sensor if there is no signal. I have signal, just not exactly what it calls for.

I recorded data when it did it again this morning. I will write it down and post it.

I appreciate any help. This is my daily driver.
 

kev2

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did it set the p1351? - before ersaing ck the freeze frame...

you are using a DIGITAL multimeter? DVOM....not an analog?
Looking at the GM trouble tree for this, step 3 suggests - ICM White wire, terminal B, disconnecting harness at ICM setting DVOM to AC scale note AC reading 1-4v ....

THe TPS was a smooth transition as throttle opened, holding it open and tapping did it change? idle and WOT were same on several open close trys?
 

stryped

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TPS was a smooth transition. Problem is, that code does not always appear and i have erased it. I also sometimes get a "Random cylinder misfire" I have had a cat code for awhile too. I replaced the sensor and no help. When I started having the current problem I drilled out the cat in hopes of fixing the problem if it was a plugged cat. However, the problem continues.

I did check that white wire and cranking a/c voltage on digital multi meter was 6 volts. (I did back probe it. I then checked the CKP sensor: 4 volts doing a backprobe.

Here is the data from this morning when I hit the manual trigger when it started missing. It was not bad enough to throw a code though.

DTC-cnt 0

Fuel Sys 1 CL

Fuel Sys 2 CL

Load_PCT(%) 8. 6

ETC (F) 194

Shrt Ft1 (%) -3. 9

LongFt1(%) 7. 8

Shrt FT2 (%) 1. 6

LongFT2 (%) 9. 4

MAP (in HG) 15. 9

RPM (/min) 1194

VSS (MPH) 42

Spark ADVC -30

IAT (F) 37

MAF (1b/min) 3. 023

TP (%) 14. 1

02sB1S12—B2S12—

O2B1S2 (v) 0. 840

SHRTFTB1S1 (%) -. 8

O2B1S2 (v) 0. 230

O2B2S1 (V) 0. 200

SHRT FT B2S1 (%) 1. 6

O2B2S2 (v) 0. 825

OBD OBD2
 

kev2

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You say you back probed terminal B white wire ?
GM suggests to ck - DISCONNECTED and AC scale ? An academic question MAYBE but lets follow their test.... Have you checked the PCM harness connectors- any work there.... always shake the harness a loose connector, rubbed thru wire, shorted etc

to be sure only codes are P0420, random P0300, ant this P1351 random also?
 

stryped

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Yes. I have shaken wires. Traced and verified grounds. I cant remember but I did the test according to the manual. And I did get 6 volts and it called for 1-4. Tested for continuty between the connector to the ignition control module and the end of the connector on PCM. Could the CKP be causing this?

Anyway not sure what else to do. Do the numbers I posted look ok?

BTW my fuel pressure still drops after KOEO but not nearly as bad as before the fuel pump change. It is within spec now. (Drops less than 5psi in 5 minutes. )

Only codes I have ever gotton were the bank 2 sensor 2 cat below threshold, the ignition control circuit voltage too high and the multiple cylinder misfire. I have only gotten the missfire one once.
 

stryped

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Yea I think it was disconnected so the vehicle would not start and I am positive it was ac scale.
 

kev2

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A test for this - do not understand it but its easy - with codes cleared, unplug the injector connector, crank for 30seconds.....does P1351 set?

6volts is deffinately high, If it sees over 4.9 sets DTC- thinking of reasons- Are secondary wires routed away from harness, as mentioned an rubbed thru wire? Will do some checking...and post

Don't see how as the CKP is seperate no shared wires- BUT then the 4v reading there...
 

stryped

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Thing is I rarely see that code, only ocassionally.

Where is the injector harness plug? DO I have to remove the upper intake?

Were those scanner numbers ok?
 

billr

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What caught my eye in the live-data was the negative ignition advance numbers. I know we have kicked this around in some other threads, debating what is "+" and "-", but generally ignition advance should never be after top-dead-center. If "Spark ADVC -30" means after top-dead-center, then that is a big clue...
 

kev2

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Page 2 Dia aids

the referance point TDC is 0 ....
after TDC (retard) is + degrees, reads as tdc + 10 ...
Before TDC (advance) is - degrees, read as tdc -10....

did I make it complicated .
 

billr

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No, what makes it complicated is the scan-tool reporting it as "ADVC"; to me that means "advance" but apparently a higher ADVC value means less advance???
 

kev2

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I read it as 30deg BEFORE tdc ......or TDC -30degrees, firing at TDC would be 0 deg, after tdc +,

I do not know the exact verbage of Acronym - hopefully some will post it- Nick maybe?
 
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