1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

[cdavy000]

Hello everyone I thought I should ask before I asked a mechanic to diagnose for me. I am very acquainted with this engine so any specific or technical answers most likely won’t be lost on me. So I’ve been having carb backfiring/ hesitation under heavy acceleration (no problems slow) only from idle and it is a brand new carb, this problem was going on even with the old carb. So I hooked up my vacuum gauge to the intake manifold and when I set the timing to 4deg btdc, fuel mixture to 1 1/4 turn, and in gear rpm of 750 I get a quickly fluctuating vacuum of around 10”-15” of Hg (can bounce between 6-18 at the extremes) while at idle. Seeing as I should be much closer to 22 I know this isn’t right and might explain why I am having hard starting condition after running for a while as well.

Side note, idle temp is 200 degrees I'm pretty sure that’s okay because it’s a fresh water cooled engine but I probably need a new impeller as it hasn’t been replaced in years. I also discovered that the engine stalls if I push the coolant hose running to the intake manifold up or down a tiny bit so don’t know what that’s about lol (not that big of a deal not like I’m constantly pushing on it)

So before I open up the valve cover to inspect valves and try to search for bent and/or burned rods or checking timing chain I wanted to see if maybe someone had a less invasive fix, thanks guys!

 

[Scott06]

Hello everyone I thought I should ask before I asked a mechanic to diagnose for me. I am very acquainted with this engine so any specific or technical answers most likely won’t be lost on me. So I’ve been having carb backfiring/ hesitation under heavy acceleration (no problems slow) only from idle and it is a brand new carb, this problem was going on even with the old carb. So I hooked up my vacuum gauge to the intake manifold and when I set the timing to 4deg btdc, fuel mixture to 1 1/4 turn, and in gear rpm of 750 I get a quickly fluctuating vacuum of around 10”-15” of Hg (can bounce between 6-18 at the extremes) while at idle. Seeing as I should be much closer to 22 I know this isn’t right and might explain why I am having hard starting condition after running for a while as well.

Side note, idle temp is 200 degrees I'm pretty sure that’s okay because it’s a fresh water cooled engine but I probably need a new impeller as it hasn’t been replaced in years. I also discovered that the engine stalls if I push the coolant hose running to the intake manifold up or down a tiny bit so don’t know what that’s about lol (not that big of a deal not like I’m constantly pushing on it)

So before I open up the valve cover to inspect valves and try to search for bent and/or burned rods or checking timing chain I wanted to see if maybe someone had a less invasive fix, thanks guys!

That temp is too high , likely you have blown a head gasket as 3.7 s are very sensitive to over heat due to open deck block design.

do a compression test , if ok have you checked ignition timing?

replace impeller make sure heat exch is clean. Do you have milk in oil ?

 

[Dubed]

Yes, 200 is way too hot. You have likely caused damage. Check the general health of your motor like Scott suggested. Pull the drive, check your impeller, flush the heat exchanger, pressure check your coolant system. If impeller has pieces broken off, find them, they may be cauing restrictions. If you ever see this motor approaching 170 -175, theres a problem. Shut it down immediately or suffer concequences!

 

[kenny nunez]

In addition to Scott & Dubed‘s suggestions you may have a worn exhaust cam lobe or a broken intake valve spring. Remove the valve cover and the spark plugs, turn the engine by hand and watch the rocker arms.
Also possible to have a cracked distributor cap causing a crossfire.

 

[cdavy000]

That temp is too high , likely you have blown a head gasket as 3.7 s are very sensitive to over heat due to open deck block design.

do a compression test , if ok have you checked ignition timing?

replace impeller make sure heat exch is clean. Do you have milk in oil ?

Welp all right then looks like she’s going back on the trailer lol. I’ll do a compression test today and post results, yes I have checked ignition timing it’s 4 deg btdc as per the Mercury service bulletin for this engine. Both oil and coolant are clean with no milk although I’m sure it’s about time for a change on both (winter project). I’ll have to replace the impeller next week when I get it back on land.

 

[cdavy000]

Yes, 200 is way too hot. You have likely caused damage. Check the general health of your motor like Scott suggested. Pull the drive, check your impeller, flush the heat exchanger, pressure check your coolant system. If impeller has pieces broken off, find them, they may be cauing restrictions. If you ever see this motor approaching 170 -175, theres a problem. Shut it down immediately or suffer concequences!

I’ll make sure to pressure test the cooling system today and post results thank you

 

[cdavy000]

In addition to Scott & Dubed‘s suggestions you may have a worn exhaust cam lobe or a broken intake valve spring. Remove the valve cover and the spark plugs, turn the engine by hand and watch the rocker arms.
Also possible to have a cracked distributor cap causing a crossfire.

I will make sure to do that and I’m sure a broken intake valve spring will be easy to see but how might I see a worn exhaust cam lobe? Do I have to open it up further? Also it’s funny you mention the cap because I recently replaced rotor, points, condenser, springs and cap on the distributor (I had the backfiring problems and other stuff before I replaced these so I don’t think my replacement is the problem) points are adjusted to spec but I did sort of test different spring combinations until I found the combo that ran best (two heaviest springs) so not the most exact approach. While I was installing new cap I screwed in the screw a little bit too much and formed a nice crack in about a 1/2 inch tall half moon arc around the screw. That little moon shape around the screw broke off the screw hole so I applied some trusty jb weld and slapped it back on, since the crack couldn’t be farther from the points on the top of the distributor cap. Although, when I’m adjusting the distributor with my hand on the metal base sometimes I get a small shock if my fingers graze a worm clamp.

 

[Scott Danforth]

you ran it to 200F. you already spit the head gasket. they spit their head gasket if the temp goes much over 175

best you can do is do a compression test and hope you didnt score the pistons. BTW, these motors have high cranking compression. you should see 170PSI on the gauge.

if you lost a cam lobe, pull the rocker shafts and push rods, remove the side cover and front timing cover, pull lifters and pull cam. (easier with motor out of the boat)

 

[cdavy000]

you ran it to 200F. you already spit the head gasket. they spit their head gasket if the temp goes much over 175

best you can do is do a compression test and hope you didnt score the pistons. BTW, these motors have high cranking compression. you should see 170PSI on the gauge.

if you lost a cam lobe, pull the rocker shafts and push rods, remove the side cover and front timing cover, pull lifters and pull cam. (easier with motor out of the boat)

Well I hope not but we’ll see. Posting compression results later today, yeah hopefully I have all my cam lobes lol

 

[cdavy000]

I have taken the valve cover off and I discovered that about 7 out of 8 screws on the cover were barely tightened down and a couple I could quite literally unscrew with my fingers. But here are the pictures I have of the rods and rockers, no blatantly obvious problems except some weird rust like growth on the sides of the rockers which I am hesitant to call rust as a good bit of it comes off with a paper towel, but it sure looks like rust. There doesn’t seem to be any “rust” near any moving joints springs or contact points it’s just kinda grown a little on the sides. A little bit of steel wool and paper towel could easily remove it tho. Springs look tip top. Should I remove the rockers to inspect further?

The rockers move excruciatingly slow whenever I crank it by hand could I just unplug the ignition coil and crank it with the starter to check movement?

Photos attached below

 

[cdavy000]

I’m not sure what this metal tab is directly above the oil hole but it’s slightly off center compared to the other ones. (Left)

Reference (Right)

A couple others arent 100% straight either, almost like they are bent a tiny bit?

 

[Scott06]

The rockers move excruciatingly slow whenever I crank it by hand could I just unplug the ignition coil and crank it with the starter to check movement?

Yes take the plugs out and ground the coil wire (take dist end off the cap) and ground. Spin it on starter. You want to look for even movement on every valve. Intakes and exhaust may have different lift amounts but all intakes and all exhaust should move about the same.

The clips on the rockers are to direct the oil so it doesnt splash too bad.

 

[cdavy000]

Yes take the plugs out and ground the coil wire (take dist end off the cap) and ground. Spin it on starter. You want to look for even movement on every valve. Intakes and exhaust may have different lift amounts but all intakes and all exhaust should move about the same.

The clips on the rockers are to direct the oil so it doesnt splash too bad.

Okay so I forgot to take the spark plugs out but I grounded the coil and this is the video I got of the valves, I can’t really see a problem but if I’m being honest I definitely wouldn’t have the eye for one.

I tested the pressure on the coolant cap and it is in fact leaking pressure… it takes about 8 seconds to lose all pressure and get to 0 psi when I pump it to 14. I can hear it leak as well. The system itself seemed to hold pressure just fine.

Thanks for the tidbit about the clips.

Video of valves moving attached below. I did it twice.

 

[cdavy000]

Just finished compression test and numbers are as follows
Cyl #1 - 126
Cyl #2 - 103
Cyl #3 - 114
Cyl #4 - 114

Engine was brought to operating temp before removing spark plugs to begin test. Ignition coil was grounded, all plugs were out, throttle neutral button pressed and then opened to WOT position, each cylinder cranked until highest psi was achieved.

 

[Scott06]

Just finished compression test and numbers are as follows
Cyl #1 - 126
Cyl #2 - 103
Cyl #3 - 114
Cyl #4 - 114

Engine was brought to operating temp before removing spark plugs to begin test. Ignition coil was grounded, all plugs were out, throttle neutral button pressed and then opened to WOT position, each cylinder cranked until highest psi was achieved.

cyl 2 is basically dead cylinder or close to it 3 &4 are not far behind.

Videos looked fine based on what you saw with pressure test I would guess head gasket. Since you could loose cooling system pressure other places besides head gasket Get #2 up to tdc with valves closed put compressed air in and see if you get it coming out the cooling system.

If yo had a leak down tester would use it to verify the valves are ok. Otherwise with cylinder at TDC and valves closed put compressed air in combustion chamber nd see if you get any air into intake or exhaust. If you need to take head off for gasket, verify its ok before putting it back on.

I think @Scott Danforth or @nola mike can tell you whose head gaskets work on this engine (OEM?) and if you will have valvetrain geometry issues if you have to shave the head when it is off.

I think what you will hear is that the 3.7/470 etc while a powerful for its size engine has some short comings

 

[cdavy000]

cyl 2 is basically dead cylinder or close to it 3 &4 are not far behind.

Videos looked fine based on what you saw with pressure test I would guess head gasket. Since you could loose cooling system pressure other places besides head gasket Get #2 up to tdc with valves closed put compressed air in and see if you get it coming out the cooling system.

If yo had a leak down tester would use it to verify the valves are ok. Otherwise with cylinder at TDC and valves closed put compressed air in combustion chamber nd see if you get any air into intake or exhaust. If you need to take head off for gasket, verify its ok before putting it back on.

I think @Scott Danforth or @nola mike can tell you whose head gaskets work on this engine (OEM?) and if you will have valvetrain geometry issues if you have to shave the head when it is off.

I think what you will hear is that the 3.7/470 etc while a powerful for its size engine has some short comings

Oh trust me I am well educated in the shortcomings at this point . Do you believe that the head gasket is the only problem?Would you or anyone else have any tips for removing and replacing head gasket?

 

[Scott Danforth]

Use a factory head gasket

Measure the head for flatness with a machinist ground bar.

If it isn't flat, get it machined

If you shave your head, you need to remeasure your push rods

 

[cdavy000]

Use a factory head gasket

Measure the head for flatness with a machinist ground bar.

If it isn't flat, get it machined

If you shave your head, you need to remeasure your push rods

All right sounds good thank you

 

[cdavy000]

So back to the carb backfiring problem and hesitation on acceleration. Is it possible that the head gasket is leaking between cylinder 1 & 2 which is causing low compression in one and high compression in the other. And would this leak cause the carb to backfire only under heavy acceleration? Thanks for the help everyone

 

[Scott06]

So back to the carb backfiring problem and hesitation on acceleration. Is it possible that the head gasket is leaking between cylinder 1 & 2 which is causing low compression in one and high compression in the other. And would this leak cause the carb to backfire only under heavy acceleration? Thanks for the help everyone

Find out if you have a valve leaking past seat that would be my inclination with low compression and backfire