Engine backfiring through throttle body when trying to accelerate

[badrano]

The engine was experiencing some backfiring through the throttle body when trying to accelerate and at times sounded like it wanted to stall out at idle. I replaced the tstat on Friday and fired it up, got it up to temp, cracked the throttle a little bit...maybe 800-1000 rpms for a few seconds just to get a little more water pressure flowing through the housing to check for leaks.....it ran fine. Today, take it out on the water....poof. Get home and on muffs, it behaved as described below.

Boat running on muffs, here's what I've captured for data from Diacom and other tools
- Fuel pressure around 43 psi when key is turned on
- Fuel pressure runs around 38 psi at idle
- RPM roughly 600 +/- a bit
- MAP sensor around 8.7 at idle
- Spark advance varies around 4 to 14 deg at idle
- Vac gauge hovers/fluctuates a little in the 30 +/- a few. I did a reading one time last year when everything was running good and saw the same behavior
- From Diacom, with the throttle cracked around 3% (not in gear), it starts to surge
-> MAP fluctuates between 11.61 and 7.81
-> RPM fluctuates between 430 to 795
-> Spark advance drops to around 4 deg during the surging
-> Vac gauge fluctuates between 32 and 17
-> No noticeable change in fuel pressure

Thoughts?

 

[alldodge]

Have you replaced the distributor cap and rotor?

Are your Vac gauge readings in kPa?

 

[badrano]

Have you replaced the distributor cap and rotor?

Are your Vac gauge readings in kPa?

My bad on the vac readings...... those numbers I posted were in cmHg.
To revise them....
At idle around 12 inHg.
When surging, 7 to 12.5 in Hg.

That is one thing I didn't check yet when I got home last night. But cap, rotor, wires were replaced in 2023, plugs replaced last season after resolving the rich condition.

 

[alldodge]

The caps on these motors may last a long time and others have a very short life. You can't look at one and see that it's bad. Search the net and you'll find they go out without warning. Yours may be fine but only one way to find out

If cap is good I would do a compression test

 

[Lou C]

Might be a vacuum leak with the surging idle and low vacuum test results; should be higher than 12” of mercury.

 

[badrano]

Might be a vacuum leak with the surging idle and low vacuum test results; should be higher than 12” of mercury.

It's not surging at idle, only when I add a little throttle. Otherwise the engine sounds/feels normal when at idle.

Last year when I addressed my fuel pressure issue, I replaced all the vacuum lines. My vac readings came from inserting the vac gauge right at the port on the intake manifold before all the other components that need vacuum. Are you saying that if I have vacuum leak, it's not due to the vacuum lines I replaced? Is this where allDodge is recommending a compression test if the cap/rotor is not the issue? Just to throw this out....engine only has 350 some hrs on it.

For the 12" of vacuum, I noted this same reading in a post a couple years back and some of the answers I got was that marine engines don't pull the same level of vac as car engines do.

 

[badrano]

The caps on these motors may last a long time and others have a very short life. You can't look at one and see that it's bad. Search the net and you'll find they go out without warning. Yours may be fine but only one way to find out

If cap is good I would do a compression test

Just found a post where some folks treat cap/rotor like the water impellar....replace it every couple years.

 

[alldodge]

Back firing through intake is caused by spark happening at wrong time, intake valve not fully closed.

It's suggested to keep a spare cap in the boat and wait for issues to start that are unexplained easy. Swap out and if it's fixed all good, if not dig further

 

[badrano]

Just an off the cuff question.....in the case of my engine with a distributor, how does the ECM advance or retard timing when you have a fixed rotor rotating around making contact with the "points" on the dist cap? I'd think you couldn't advance or retard too much otherwise the rotor won't be in contact with a given "point" on the cap?

 

[alldodge]

Unable to say exactly but controlled by the ECM. The contact width of the rotor and width on cap provides a certain amount of degrees

 

[Scott Danforth]

ECM reads the pulse from the trigger, and then fires the coil. It can vary the timing +25 degrees to -15 degrees