Cylinder 3 misfire and low/erratic fuel pressure

AdrianMS6

AdrianMS6

Greenie N00B Member
Greenie Member
My MS6 has developed a misfire on cylinder three and threw both p0300 and p0303 codes.

The spark plugs are all good and gapped within factory spec, I tried checking the coil packs. But after clearing the code to see if the cylinder switched along with the coil pack, the check engine light has not come back on though it is clear the car is running poorly and still misfiring.

I borrowed a nicer scanner to run some diagnostics and found the car has really low fuel pressure. It's bouncing between 60 and 200 psi on idle and only got up to 1400 at WOT. I spliced in a fuel pressure gauge to check the in-tank fuel pump as I have just changed that out for a Deastchwerks one and I was getting ~60 psi before the HPFP.

I've read a little about the fuel pressure relief valve causing issues and a cylinder 3 misfire, does that seem like the culprit here? Or is it probably the HPFP itself?

Any advice is appreciated.
 
Your pressures post ITFP are low. Do not WOT the car till you have repaired the issue.
Here is a good read about the HPFP and how it works...
CS HPFP Troubleshooting
It is possible that your previous issue was related to this issue and it would be valuable to test the whole system and consider replacing and upgrading. Seeing as this could be a spill valve, relief valve issue or mechanical issue you should run some tests and look into pulling the pump apart. The spill valve can be cleaned and inspected. Now would be the time to check for damaged internals or upgrade if stock.
Does your car have upgraded internals?
With the age of these cars, Im finding tasks are switching from repairs to rebuilds.
 
SyntheticAtmosphere said:
Your pressures post ITFP are low. Do not WOT the car till you have repaired the issue.
Here is a good read about the HPFP and how it works...
CS HPFP Troubleshooting
It is possible that your previous issue was related to this issue and it would be valuable to test the whole system and consider replacing and upgrading. Seeing as this could be a spill valve, relief valve issue or mechanical issue you should run some tests and look into pulling the pump apart. The spill valve can be cleaned and inspected. Now would be the time to check for damaged internals or upgrade if stock.
Does your car have upgraded internals?
With the age of these cars, Im finding tasks are switching from repairs to rebuilds.
Click to expand...

Yeah, I'm not gonna be driving the car unt I get this all sorted out. I don't want to break it more.

I just replaced the ITFP and fuel pump relay, which seemed to fix my previous issue. I haven't had any issues with the ITFP since.

The car is practically stock, so no upgraded internals.

That's definitely a good read and very helpful, so thanks for sharing that. I'll see what else I can test, and I will definitely be pulling the HPFP apart soon.

Do you have any recommendations for upgraded fuel pump internals? I am in CA so sadly have to stick to CARB legal stuff.
 
Internals arent affected by CARB. However to fully benefit from them you need to adjust the max pressure via tuning.
I have Autotechs, because they were the gold standard at the time, but any reputable brand is good. Corksport also has them.
 
SyntheticAtmosphere said:
Internals arent affected by CARB. However to fully benefit from them you need to adjust the max pressure via tuning.
I have Autotechs, because they were the gold standard at the time, but any reputable brand is good. Corksport also has them.
Click to expand...

That's good to know. I'm coming from a pre-1975 project car where emissions didn't matter at all, so I am still trying to wrap my head around what I need to worry about and what I don't when it comes to CARB.

Thanks for all the advice, I appreciate it.
 
I have the same exact issue. I have the fuel rail off and was gonna swap injectors around to test as a last result but just figured out I can blow through both ends of my relief valve. That means it’s toast right?
 
Haedenjames said:
I have the same exact issue. I have the fuel rail off and was gonna swap injectors around to test as a last result but just figured out I can blow through both ends of my relief valve. That means it’s toast right?
Click to expand...
Unless you somehow blow at >1800 psi then yes it's toast
 
So how the hell does it cause a misfire on only cylinder 3 I’m so confused. Shouldn’t it affect all cylinders?
 
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