No speedo car sputters

WA08TC

Well-Known Member
Upon acceleration the car spits and falls on its face when I can get enough speed it will cruise fine. At the same time I noticed this I noticed the car doesn't show the speed but everything else works fine. Anyone have an idea besides replacing the VSS?
 

WA08TC

Well-Known Member
I haven't it happened on the way to work and I'm here currently til the morning. I can get the car home.
 

Ross

Goblin Guru
DTC P0502 is where I would start. Either bad wires, or bad sensor.

Circuit/System Testing

1. Disconnect the ECM harness connector X1.
2. Measure for 1,300-1,500 ohms at 25°C (77°F) between the VSS high signal circuit terminal 54 and the VSS low signal circuit terminal 34.

• If the value is greater than 1,500 ohms test the VSS high signal circuit and the VSS low signal circuit for an open/high resistance. If the circuits/connections test normal, replace the vehicle speed sensor.

• If the value is less than 1,300 ohms test the VSS high signal and the VSS low signal circuits for a shorted together condition. If the circuits/connections test normal, replace the vehicle speed sensor.

3. Measure for more than 10 ohms of resistance between the VSS high signal circuit terminal 54 and ground.

• If the value is less than the specified range, repair the short to ground.

4. Measure for more than 10 ohms of resistance between the VSS low signal circuit terminal 34 and ground.

• If the value is less than the specified range, repair the short to ground.

5. Reconnect the ECM connector. Disconnect the VSS connector.
6. Ignition ON, test the VSS high signal circuit terminal B for a short to voltage.

• If all circuits/connections test normal, test or replace the VSS.
 

WA08TC

Well-Known Member
Could the VSS somehow cause the misfires and the car to run like crap? Or do I have a second issue? 1 thing I forgot to add that points toward the bad VSS also is the past couple times starting the car cold it would struggle to idle for about a minute then be good.
 

Ross

Goblin Guru
Good point. I was confusing the VSS for the crank position sensor. My LSJ has the vehicle speed sensor in the F35 transaxle, which should be unrelated to an engine misfire. The crank sensor does cause misfires, as I have had to fix that on my LSJ.

Well then, the P0300 now looks like a better starting place.

Circuit/System Testing

1. Verify that the following conditions do not exist:
- Engine vacuum leaks.
- Fuel pressure that is too low or too high--Refer to Fuel System Diagnosis.
- Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
- Restricted exhaust system--Refer to Restricted Exhaust.
-> If you find any of the above conditions, repair as necessary.
2. Ignition OFF, remove the ignition coil from the misfiring cylinder, leaving the electrical connector connected.
3. Install the J 26792 to the boot of an ignition coil and ground.

Important: An erratic or weak spark is considered a no spark condition.

4. Attempt to start the engine and observe the J 26792 . The spark tester should spark.
-> If there is no spark, refer to Electronic Ignition (EI) System Diagnosis for diagnosis of the ignition coil.
5. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the following conditions do not exist with the spark plug:
- Gas, coolant, or oil fouled.
- Cracked, worn, incorrectly gapped--Refer to Spark Plug Inspection.
-> If there is a condition with the spark plug, replace the spark plug.
6. Exchange the suspected spark plug with another cylinder that is operating correctly.
7. Operate the engine under the conditions in which the misfire occurred. Observe the Misfire Current Counters with a scan tool. The misfire should not follow the spark plug exchange.
-> If the misfire follows the spark plug, replace the spark plug.
8. If all conditions test normal, test or inspect for the following:
- A lean or rich fuel injector--Refer to DTC P0201-P0204, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P2146, P2149, P2152, or P2155.
- An engine mechanical condition--Refer to Symptoms - Engine Mechanical.
 
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Gtstorey

Goblin Guru
All cylinders misfiring would push toward a fueling issue or an airflow/MAF/MAP issue. But it could also be a wiring issue on the coils or injectors.
 

Tinkles

Well-Known Member
Just my $.02.

1. I might be able to believe that a faulty VSS could be making the ECU think that you are hitting the speed limiter when you are not. But I believe the factory speed limiter pulls fuel not spark, so I cannot be sure that it would be causing the misfire. Also, with it only misbehaving under acceleration does make this idea less likely.

2. Does the wiring for the ignition coils and VSS ever run inside the same section of wire loom or run close to each other? If yes, I would inspect it for damage. Are they grounded to the same spot?

3. My 1st go-to check with a boosted car misfiring under acceleration is spark plugs. How old are they? What is their gap? Are they wet after the engine was running? Are they clean? Or do they have carbon buildup? I personally start around .035" for gap and start closing the gap if needed. I would expect this to be a constant issue, but checking the plugs is free/easy.

4. I'll 2nd that it could also be a fueling issue. It is possible that something happened with all 4 injectors, but I think that the fuel pump would be a more likely problem than all 4 injectors failing. Is there any way to view the fuel pressure when you are accelerating? Is the fuel filter clogged? Does the wiring for the VSS and fuel pump or injectors run together in the same loom? Are they on the same ground? Maybe a bad tank of gas?

5. I have dealt with a faulty MAF before on one of my cars, and it was not happy at all while it tried to accelerate. I forget if or what codes it logged.

6. Possibly charging issue? Alternator dying and not supplying enough power to the fuel pump under acceleration? I see this as an unlikely chance.


I'm just spit-balling ideas and trying to think of anything that I think that could fit.



What about an issue with the cluster? Thoughts, anyone? I do not know how important the cluster is to the whole electronics system/ECU. It seems like a .00000000001% chance of it being an issue, but I am unfamiliar with how it interacts with the ECU. The only thing I do know on this is that GM has had an issue in the past with gauge clusters failing, but I did not hear of any instances where a failed cluster affected another system in the car.
 

WA08TC

Well-Known Member
So the car idles fine not hunting, vacuum is at 17hg so good there. If I jump on the throttle real quick she goes fine so I don't think it's fuel. Pulled the VSS out just now cause it's simple after a 13hr shift. That is most likely my speedo issue. Also have to put the brake pedal on the floor to get grabby so maybe brake booster line came off in front? But wouldn't that cause the idle to be irratic? I'm going to swap out the VSS today since they have it in stock down the street and go from there. Plugs are about a month old 41-108 AC Delco at .32
IMG_20240924_070442156.jpg
 

Rttoys

Goblin Guru
Since the brakes are goofing up and you are thinking booster, block off the vacuum at the engine for the booster and see what happens.
 

Markm

Well-Known Member
I see the sensor has a clean spot. Does it appear the reluctor could be rubbing on the sensor? If so that would be a sign of worn bearings. Could also drain the fluid and see how much metal is in it
 

Robinjo

Goblin Guru
My speed sensor wires were broken and caused my speedometer not to work. Luckily it was right by the connector so I was able to just replace the connector end. It also threw the code P0502 but ran fine, just no speedometer function. It also gave me the warning 'Power Steering' issue on the dash.

I also one time forgot to hook up my vacuum line (or it came unconnected) from the brake booster and it ran like garbage. That could 100% cause your Misfire problem. It idled ok out of the driveway but was terrible upon any throttle beside light cruising.
 

WA08TC

Well-Known Member
I cleaned off the sensor a bit but I'll drain. I'm hoping the brake boost line came off at the booster, that would be a relief but tomorrow is my day off so I'll take the hood off and take a look.
 

WA08TC

Well-Known Member
Replaced the VSS fixed the speedo issue. Haven't driven it yet but not giving me any codes for misfire at idle. Soft brake pedal is the rear brake tee on the driver side. Found a snail trail in the garage all the way to the door. I'll have to bleed again. I unscrewed the fitting, put some thread locker on it and screwed it back in. It was decently tight prior but wrenched it down again with the 12mm, but hopefully it will fix it.
 

WA08TC

Well-Known Member
Update. Car still shows misfire cylinder 1 and 2. No more VSS fault, speedo works, car runs likebits suppose to, boost, vac, afr all where they are suppose to be.
 

Markm

Well-Known Member
Did you happen to replace the timing chain? If it’s running correctly you may need to try a crankshaft correlation relearn. Could try it even if you didn’t replace the timing chain and see if it still counts misfires.
 

WA08TC

Well-Known Member
I hooked up HP tuners to it cylinder 1 and 2 were counting fast up at idle almost identical. 3 and 4 were good. The fuse box and starter were only seeing 5v. Then I was trying to diagnose and car wouldn't power on, like nothing. Dead. Like wtf all this stuff at once, came to find out my battery terminal was loose and needs to be replaced, when I twisted it to check every thing powered on. Hoping somehow the coils were upset about not getting full juice and when I replace the terminal tomorrow everything will be good. I mean it was while I was tuning and everything but all the vibrations from the car wiggled it loose.
 
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