8.1 gi-b misfire / backfire

[Fedds]

8.1l gi-b chapparal 26ssi 2002. I'm pulling my hair out here. Idle seems a bit rough, gets way worst with throttle advanced. Will pop / backfire.
Compression all cylinders at or near 150psi. Replaced spark plugs. A few looked Carbon-ed up. Diacom power balance test shows 6 and 8 are not producing power (best I can tell). Timing light on all wires shows consistent spark, but I replaced those two coil packs anyways. No change I performance.

Fuel pressure right at 40 psi on the fuel rail the whole time.

Figured its got to be injectors. Thought I cross referenced to the right GM injector, but it must not have the same flow rate as I replaced them all and now it's even worse. Finger on injectors says they are all clicking. Gonna put the old ones back in.

No codes. Diacom also shows idle MAP around 16mmHg (this boat is new to me so I have no idea what is normal) . Timing advance in software seems to be all.over the place. Is that normal at idle?

What am I missing here?

 

[dubs283]

16mm/hg would be about 0.75in/hg which if is vacuum very low for idle. MAP is pressure so not sure if the scale is correct. That said, if you unplug the MAP sensor what happens.

Would be good to see if TPS is reading correctly, set diacom to live data and see if the % throttle reads accordingly

 

[Fedds]

Key on engine off I've the throttle through the range of motion and it went from 0 to 100 % pretty smooth.

Not sure what happens when you unplug the map. I assumed the speed density map had to have that to run?

 

[dubs283]

I assumed the speed density map had to have that to run?

Maybe, not super familiar with that engine but it's something to try. Most marine efi systems are not that complex, especially non EC engines. If a sensor reads out of range the ecm will still try to keep the engine running based on info from the other sensors. It won't run perfect but it'll get you back to the dock

Unplugging the MAP sensor should trigger a fault, most likely a circuit high/low. If the sensor is faulty and giving false readings that are within range when it's plugged in unplugging it may alleviate symptoms to a degree

 

[Toddavid]

Is this a closed loop system with O2 sensors? If so, what’s your short term fuel trim look like on each bank when warmed up?

 

[Scott Danforth]

Your fuel pressure is low. Get the injectors flow tested

 

[Fedds]

Is this a closed loop system with O2 sensors? If so, what’s your short term fuel trim look like on each bank when warmed up?

Not closed loop. No O2 sensors (atleast pretty sure)

 

[Fedds]

Your fuel pressure is low. Get the injectors flow tested

Really, I thought 8psi on the low pressure pump and 40 on the high pressure pump, all plus or minus 4 psi was the spec. What do you think it should be?

Totally agreed on flow testing injectors

 

[dubs283]

Your fuel pressure is low. Get the injectors flow tested

40 psi out the high pressure pump would not cause this much poor performance, maybe some top end faltering but OP says this thing idles poor and immediately backfires/pops.

Possible injector issue, an 8.1 will run on four cylinders. Very poorly but still run.

Have you done a cylinder drop test? Fingers on injectors to feel pulse is one thing but if you disconnect the leads one at a time with the engine running you should note an even drop in rpm. More noticeable at higher rpm

 

[Scott Danforth]

I thought the 8.1 spec from volvo was closer to 45 psi on the high side and 8 psi on the low side.

 

[paulswagelock]

Chased this for months on a friends Merc version of this motor. Final result - found 2 injectors plugged.

 

[dubs283]

I thought the 8.1 spec from volvo was closer to 45 psi on the high side and 8 psi on the low side.

It may be that exact spec, all I'm saying is a fuel pressure of 40 psi vs 45 psi for high pressure would not be suspect for OP's issue.

If in fact the psi is dropping severely when throttle is advanced then yes I would be suspect, however OP has not shared that info

 

[Fedds]

Chased this for months on a friends Merc version of this motor. Final result - found 2 injectors plugged.

I'm down to this. I bought a gm cross reference injector set and it ran worse with same symptoms. Turns put volvo put like 42lb/ht injectors on it and gm ran something like 26 or 36 lb/hrs. Injectorsnarenbeing sent out for cleaning and balance and flow test. I destroyed one getting it out.

 

[Scott Danforth]

I'm down to this. I bought a gm cross reference injector set and it ran worse with same symptoms. Turns put volvo put like 42lb/ht injectors on it and gm ran something like 26 or 36 lb/hrs. Injectorsnarenbeing sent out for cleaning and balance and flow test. I destroyed one getting it out.

yes, the marine motors get different fuel systems than the automotive versions. Reylar has injectors if you need.

 

[Fedds]

It may be that exact spec, all I'm saying is a fuel pressure of 40 psi vs 45 psi for high pressure would not be suspect for OP's issue.

If in fact the psi is dropping severely when throttle is advanced then yes I would be suspect, however OP has not shared that info

I checking it under load when the issue was happening. Stayed at 40 psi. Granted, I cant run it up to high RPM with this issue so I can't check it there yet, but when thus backfire / miss happens fuel pressure is 40 psi.

Trust me, I was hoping this was it.

 

[Fedds]

Update and issue solved. Sent injectors out for cleaning since all diacom data seemed reasonable other than the iac was darn near wide open trying to keep it running and the timing light said it was sparking. Has to replace one injector, and don't try to cross reference to a gm part number. Alot of people specify a lb/hr or cc/mm but don't specify out the line pressure for that flow. Gm runs like 58psi, we run 40. Anyways, Runs like a top.

If anyone does this, the injector removal and swap is not hard, unless you are like me and the fuel rail is corroded bad where you have to pull the injector past. I cut a few O rings and Grease to help get them in the fuel rail and intake ports is vital. Go slow, a new fuel rail, like these injectors, is hard to come by and expensive. Triple check for fuel leaks. There is one connector on the fuel rail for the injector harness, a vacuum line to the fuel pressure regulator on my model, and the map sensor connector hard to be removed. Lots of fuel line so I left mine connected and just flipped it over to work on it right on top of the engine.