Misfire / bucking under load, no code?

Mattcomet82

Registered User
This just started happening yesterday to my daily, whether it is cold or warm. It's worse when the motor is under load, like immediately before it downshifts when starting up a steep incline. I did a little searching, and I saw that people recommended pulling the cam sensor to see if the problem went away. I tried it, and it wouldn't start. Next I put a scanner on it, with nothing for KOEO. It gave a code 538 for KOER test. I ran the test again, this time hitting the throttle to WOT as it instructs in the scanner manual (manual footnote says not all cars need the WOT test), and it gave a 111.

It's mostly stock, but here are a list of updates/swaps I have made in the recent past:

-K/N filter and resonator delete (4/15/16)

-Taylor Thundervolt 8.2mm plug wires (3/30/16)

-Motorcraft Fuel filter FG881 (4/15/16)

-Motorcraft Copper core plugs (part number?) (08/2015)

-All 3 belts / idler pulleys (3/3/16)

-Supercharger fluid (3/3/16)

-Bosch o2 sensors (3/3/16)

-Trans rebuild (1/2016)


I'm looking at having to take it to a shop, and I still have yet to find a mechanic that knows much about the SC cars in my area. Thanks for your input in advance!

Matt
 
Good evening


What is vacuum at idle? 17 - 20 is considered normal. IF not suspect vacuum leak.

Intercooler tubes, supercharger top due reseal.

1989 - 1993 Harmonic Balancer wobble causing vane to strike Crank Position Sensor leaving "witness" mark. Center Balancer bolt broken and or missing. The Harmonic Balancer rubber elastomer separating. See Link: https://www.google.com/url?sa=t&rct...wfKaZVPa01BKZINxw&sig2=2JS_XWcWZumXb2-wRARBZw.

Fuel pressure out of limits. Check with fuel pressure gauge.

Routing of the Cam Position Sensor wiring connector. Check for the wiring connector not rubbing against the super charger belt.


Hope this helps.


Good Luck.
 
On my way into work this morning, I noticed a considerable difference between running cold and warm. It appears my first observation was incorrect.
 
It may take several tries to start with the cam sensor unplugged as it tries to "guess" where it should be. Cam sensor has solved what you r describing for me twice. I'd try disconnecting again and giving it several tries to start

Adam
 
It may take several tries to start with the cam sensor unplugged as it tries to "guess" where it should be. Cam sensor has solved what you r describing for me twice. I'd try disconnecting again and giving it several tries to start

Adam

Before I left work today, I pulled the cam sensor and tried for a solid 10 minutes. It really wanted to, but never would start.
 
Good evening


What is vacuum at idle? 17 - 20 is considered normal. IF not suspect vacuum leak.

Intercooler tubes, supercharger top due reseal.

1989 - 1993 Harmonic Balancer wobble causing vane to strike Crank Position Sensor leaving "witness" mark. Center Balancer bolt broken and or missing. The Harmonic Balancer rubber elastomer separating. See Link: https://www.google.com/url?sa=t&rct...wfKaZVPa01BKZINxw&sig2=2JS_XWcWZumXb2-wRARBZw.

Fuel pressure out of limits. Check with fuel pressure gauge.

Routing of the Cam Position Sensor wiring connector. Check for the wiring connector not rubbing against the super charger belt.


Hope this helps.


Good Luck.


1. Vacuum gauge on the dash reads ~19hg at idle.
2. I sealed the tubes with silicone gasket in 08/2015 when I replaced the water pump, plugs, and other minor pieces. Time to redo it already?
3. I do have a wicked vibration coming from the front end that is most noticeable when parking the car. Mostly shifting between reverse and forward gears. I had previously attributed this to faulty motor mounts, and have those ready and waiting. The thought hadn't occurred to me that it may be a balancer issue. Although, this misfire issue didn't start creeping up. It started happening immediately.
4. Don't have a fuel pressure gauge, but will put that on the list.
5. I've thoroughly inspected the CPS wires, and there is no damage.
 
I stopped on the way home from work today and got a new CPS. Only difference now is that it will misfire at idle every so often. The last thing I did to the car before it started having these issues was replace the fuel filter. I'm going to pull that next and see if it will tell me anything.
 
I had the exact same symptoms you are describing. It ended up being an injector stuck open when warm. Can you do a cylinder balance test? That may point out the location of the misfire.

Maybe you had debris move and lodge in an injector when you changed the filter?
 
I pulled the air tube off and reinstalled it several times yesterday, and I think I found my issue. There was a stray piece of electrical tape coming apart from a cable run that I believe was interfering with the tube seal at the TB, causing extra air to get in. Runs like a champ now.
 
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