2005 VP 5.7GXi-FF with rough idle and popping under load

[Rickenbacker]

Hi everyone, this is my first post. I've read many posts here but not sure if found my exact issue.

The VP 5.7GXi engine has suddenly developed an occasional popping when higher RPM under load (maybe above 2-3k). It also has a slight rough idle but normal RPM. Idle sounds almost normal after it warms up. Engine sounds different and hesitates slightly when returning to idle or accelerating. Sounds like backfire on top of engine. It can be a single pop or a series of crackle pop. Power feels lower than normal and drops more when I hear pop(s). I also think I see and smell a little gas in the exhaust. I put some sea foam in tank, swapped fuel filter, replaced plugs, wires and idol air control valve (because early in the year had a high idle. But the high idle went away months ago but I thought it wouldn't hurt to replace it) Now symptoms still exist but popping may be less but still got it to do it once. I suspect fuel injectors but I’m no expert or mechanic. Someone suggested to look at the temperature sender. Other posts mentioned lean condition. Water here in Lake Michigan cooled off suddenly.

I have a couple calls into local marine mechanics but I'm not sure how quickly I'll get anyone to look at it. I may have to consider season over.

Thanks for any advice!

 

[alldodge]

Backfire at higher rpm in most cases is a lean condition. Rough idle and higher then normal would be a vacuum leak. A vacuum leak can cause both of these

What is your motor serial number?
What rpm is idle speed in gear and neutral?
What is fuel pressure at the rail?

 

[Lou C]

How did the plugs look when you removed them? Any looked steam cleaned or rust on electrodes? Because sometimes you can get a bit of water in a cyl from a leaky elbow gasket and that can cause some corrosion on valves causing them not to seat properly. This goes along with the symptom of rough idle....

 

[Rickenbacker]

Thanks for your response Alldodge.

The Engine S# is 4012156824

rpm is idle speed in gear sounded normal, gave it a quick glance yesterday, maybe 650 and neutral looked to be 650-700.

What is fuel pressure at the rail? I do not have gauge. This is my first dive into diagnosing anything myself. However, youtube videos of that look easy enough.

 

[Rickenbacker]

How did the plugs look when you removed them? Any looked steam cleaned or rust on electrodes? Because sometimes you can get a bit of water in a cyl from a leaky elbow gasket and that can cause some corrosion on valves causing them not to seat properly. This goes along with the symptom of rough idle....

Plugs looked okay, just an even matte black carbon on them, no rust. The were gapped at around .04 not the .06 from the manual.

 

[Rickenbacker]

Thanks for your response Alldodge.

The Engine S# is 4012156824

rpm is idle speed in gear sounded normal, gave it a quick glance yesterday, maybe 650 and neutral looked to be 650-700.

What is fuel pressure at the rail? I do not have gauge. This is my first dive into diagnosing anything myself. However, youtube videos of that look easy enough.

The Engine S# is 4012156824

rpm is idle speed in gear sounded normal, gave it a quick glance yesterday, maybe 650 and neutral looked to be 650-700.

What is fuel pressure at the rail? I do not have gauge. This is my first dive into diagnosing anything myself. However, youtube videos of that look easy enough.

Sorry for the double post. Just figured out the "quote button"

 

[alldodge]

Many auto parts stores loan gauges to measure fuel pressure. There is a schrader vale on the rail covered by a cap (like and valve stem cover on a car tire)

Having the HVS distributor, I would replace the cap and rotor. This may not be your issue but those caps fail and cause problems without warning. If the cap does not fix the issue, then keep it in the tool box when it does happen

 

[Rickenbacker]

Many auto parts stores loan gauges to measure fuel pressure. There is a schrader vale on the rail covered by a cap (like and valve stem cover on a car tire)

Having the HVS distributor, I would replace the cap and rotor. This may not be your issue but those caps fail and cause problems without warning. If the cap does not fix the issue, then keep it in the tool box when it does happen

The distributor is the Delco EST (Flat) I replaced that last year. I'll check it again. And get that gauge. Any idea what the pressure should be?

 

[alldodge]

Should be 43 psi at 1800 rpm

 

[Rickenbacker]

Should be 43 psi at 1800 rpm

Thanks!

I checked Advanced Auto and they couldn't find a fuel pressure gauge. Nothing to buy or loan me. While there I got a new distributor cap and rotor and popped those on before dark along with my spare brand-new fuel filter. The rotor looked fine and cap had a bit of corrosion. It still idles the same with a slight random stagger at 650-700RPM. When I rev it in neutral up to 2k no popping, same as before without a load. When I bring it back to idle quickly it sounds like it almost stalls before coming back to 650RPM. And I see gas bubbling up to the surface of the water at the exhaust, about a drop every few seconds. This is confusing my simple caveman brain. How can it be a lean condition with so much fuel not burning? Could popping be coming from exhaust manifold? The manifolds were replaced a few years back, but I worry about corrosion because it was a salt water boat for 12 of the 15 years. Maybe timing or valve issue?

I'll see if I can get that fuel gage tomorrow from another store.

 

[alldodge]

I mentioned in previous post that in "most cases" backfire is lean condition. Backfiring can also be caused by valves and flooding. If an exhaust valve is open slightly or damaged where it doesn't seal very well, this can happen. If an injector is leaking fuel then when the intake valve opens it can happen

Seeing fuel on the water may be an issue with the injectors

 

[Rickenbacker]

I mentioned in previous post that in "most cases" backfire is lean condition. Backfiring can also be caused by valves and flooding. If an exhaust valve is open slightly or damaged where it doesn't seal very well, this can happen. If an injector is leaking fuel then when the intake valve opens it can happen

Seeing fuel on the water may be an issue with the injectors

Thanks for the detail. That makes sense. When I think of injectors, I only think of clogging failure and I was not thinking about a leaking issue.

I would doubt these injectors have ever been checked, cleaned, or replaced.

 

[Rickenbacker]

Here is an update. My auto mechanic is getting into boats. We pulled the brand new plugs out and he inspected them and also did a compression test. All cylinders checked out fine. All plugs showed signs of running rich. There were no sounds or signs of bad injectors during compression test either. I think we have ruled out fuel and any compression related cause. He doesn't like just throwing new parts on but suspects that the MAP sensor or coolant temp sensor may be bad causing the engine to get way to much fuel. He suggested I replace them both and thought that the price for both should be under $100. The VP MAP sensor is 3859020 and is $280. Ouch. Anyone know of a compatible one. I found this for $44. https://www.amazon.com/RPS-Sensor-Mercruiser-3859020-3858987/dp/B07N55QJHV

Also, if anyone has any more thoughts, please let me know.

Thanks!

 

[Tristador]

Enter that MAP sensor part number on eBay. I think you'll be glad you did.
It's a common sensor

 

[alldodge]

All cylinders checked out fine.

What were the numbers?

There were no sounds or signs of bad injectors during compression test either

How does anyone know if injectors are good or bad from a compression test
Asking for a friend :facepalm:

If compression is good, and compression is close to a new motor, then the only way to get a backfire from higher rpms is either, lean condition or incorrect timing

He doesn't like just throwing new parts on but suspects that the MAP sensor or coolant temp sensor may be bad causing the engine to get way to much fuel.

If these item are causing to much fuel to be injected then there would be no backfire because the valves are seal very well because "compression numbers are good"

 

[Rickenbacker]

I'm not sure how he deducted that the injectors were likely good. He said something about not hearing or seeing something from the open plugs when we were cranking the motor.

I just realized I did not share all the info. He did say cyl 3 and 5 had slightly lower compression but he felt that would not cause the problem. I forget the numbers.

I need to figure out how I can get a diagnostics scanner on it. I've read that there is really no low cost one available for Volvo Penta. Is this true?

 

[QBhoy]

From what you’ve said...and you have definitely replaced thee single most common cause of such things..the diz cap...I’d ask if your mechanic made certain you defo have the correct iridium spark plugs in there and not the normal ones. Then I’d ask about if she has ever taken in water through the valves and later making them occasionally stick. Then I’d ask again...are you sure you replaced the diz cap. Even if it had one last year !!! They fail so easily after so little time and depending on conditions when not in use. Can’t tell it enough really. 5 caps in 5 years now.

 

[Rickenbacker]

From what you’ve said...and you have definitely replaced thee single most common cause of such things..the diz cap...I’d ask if your mechanic made certain you defo have the correct iridium spark plugs in there and not the normal ones. Then I’d ask about if she has ever taken in water through the valves and later making them occasionally stick. Then I’d ask again...are you sure you replaced the diz cap. Even if it had one last year !!! They fail so easily after so little time and depending on conditions when not in use. Can’t tell it enough really. 5 caps in 5 years now.

I replace Diz cap last year (just because it looked corroded) and Cap and rotor after this problem started (4-5 weeks ago) and I can not tell any difference.

The iridium plugs (Delco) I pulled out were .04 gapped. They were in there for at least 3 years (Before I acquired the boat). I put the same plugs in but gapped them at .06. The ones in there were .04. I know, stupid, easy to damage, weird angle etc. but I was very careful. But this is the gap my manual has. I ordered some NGK that their site claims compatible with my 5.7 GXi but they are not iridium. Not sure the Volvo/Champion ones are iridium.

Is there any strange way timing can suddenly change? I've read that gears can wear that could retard timing.

Thanks again all for all the help.

 

[QBhoy]

I replace Diz cap last year (just because it looked corroded) and Cap and rotor after this problem started (4-5 weeks ago) and I can not tell any difference.

The iridium plugs (Delco) I pulled out were .04 gapped. They were in there for at least 3 years (Before I acquired the boat). I put the same plugs in but gapped them at .06. The ones in there were .04. I know, stupid, easy to damage, weird angle etc. but I was very careful. But this is the gap my manual has. I ordered some NGK that their site claims compatible with my 5.7 GXi but they are not iridium. Not sure the Volvo/Champion ones are iridium.

Is there any strange way timing can suddenly change? I've read that gears can wear that could retard timing.

Thanks again all for all the help.

Hi there. Small chance I’m wrong, but by what you’ve said about the plugs being AC delco, they are almost certainly platinum and not the now preferred iridium. Have this in mind for a few reasons. Some of which would be that I mistakenly put ac delco plugs in mine the first year I had an mpi/gxi type engine. It ran very poorly. Other reason would be that I’m pretty sure you can’t gap iridium plugs usually too. I’d also say that you defo need iridium plugs for sure. The NGK 5599 plugs are the ones you need I’m sure.
but apart from all this and more likely an issue is that you should be aware that under no circumstances should you assume that because an mpi/gxi had its famously diz cap replaced last year, should you also assume its all good. Very very common mistake to make and well documented cause of such an issue as yours. They can fail very quickly and most commonly after Storage or lack of use.

 

[Rickenbacker]

Here is an update and a recap.
Sudden problem of popping under load.
Replaced plugs, wires, cap/rotor,and idol air control valve . Problem persisted, but maybe slightly less so.
Checked compression. Looked good. Mechanic said it was running rich from looking at all plugs and recommended replacing MAP sensor and coolant sensor. I replaced only the MAP sensor along with a different set of plugs (NGK V-Power 3951) their site matched them to my VP5.7GXI)
Engine idles smoother and no popping/backfiring under load at all or any time. Idle is slightly rough when cold. Idle fluctuates up and down from 650-1000, more so when colder. I suspect it still is running rich. Exhaust smells of gas more than normal. New plugs look black and oily/fuel covered looking.
I replaced coolant temp sensor and fluctuating idle improved did not get to take it out on the open water). Went back today to pull out for the season and cold idle is slightly rough and idle RPM still fluctuating, not noticeable when cold, but more after it warms up.
I plan to get a mechanic to look over the winter/early spring and check what I can when it is in storage (indoor/heated)

Thanks for all your help with this. If anyone has any ideas, please post.

Thanks!