'90 3.0 rough idle - vacuum gauge interpretation

[Rick Stephens]

Bought a 1990 boat with a 3.0 Merc a year ago. With a lot of assistance here have fixed repaired adjusted and made my way through many of the systems. Engine history is unknown. In general runs pretty decent. The engine has used no oil after a couple hundred hours use this summer. Looking inside each spark plug hole with a borescope shows what looks like a brand new engine. Clean hone marks, few scratches. Virtually no carbon build up. Same in the rocker cover. Absolutely pristine clean up top.

I have had some issues with water in the outdrive and the shifter killing the engine. It took basically going through every detail to get it fixed - adjusting the cable linkage exactly to spec is important. Nice to have done it, between being nose deep in the manuals, reading here and watching achris's videos, I am getting a great working knowledge of this boat.

Now I have a rough idle. At first, I thought it was a lot worse since the tach is off by several hundred - reads high. With the dash tach saying I was idling at 1100 RPM I couldn't get it to run at all trying to reduce it to 800-850. Attaching a good diagnostic tach showed me the error and I can set idle ok where I want it.

Engine feels rough at 850. So replaced plugs, checked timing, rebuilt carb and ran it pulling wakeboarders for a day. No problems except that rough idle. So hooked up a vacuum gauge. And I am stumped. I have never seen a vacuum gauge work like that in an unblown engine. I could see it with a high lift cam, but I can't believe anyone would make one for a 3.0L, let alone install one. At idle to about 1300 RPM the needle rapidly oscillates between 10 in and 15 in Hg. At 2000 rpm it settles down to 17 inches with barely a flutter.

Maybe the engine was rebuilt and they did the valve guides incorrectly?

I am going to try and post a link for a video I shot of the vacuum gauge. I have had a lot of trouble posting links, so I may not get it done. We'll see.

Youtube video https://www.youtube.com/watch?v=w_boOaxU8FM

Rick

 

[alldodge]

Bought a 1990 boat with a 3.0 Merc a year ago. With a lot of assistance here have fixed repaired adjusted and made my way through many of the systems. Engine history is unknown. In general runs pretty decent. The engine has used no oil after a couple hundred hours use this summer. Looking inside each spark plug hole with a borescope shows what looks like a brand new engine. Clean hone marks, few scratches. Virtually no carbon build up. Same in the rocker cover. Absolutely pristine clean up top.

I have had some issues with water in the outdrive and the shifter killing the engine. It took basically going through every detail to get it fixed - adjusting the cable linkage exactly to spec is important. Nice to have done it, between being nose deep in the manuals, reading here and watching achris's videos, I am getting a great working knowledge of this boat.

Now I have a rough idle. At first, I thought it was a lot worse since the tach is off by several hundred - reads high. With the dash tach saying I was idling at 1100 RPM I couldn't get it to run at all trying to reduce it to 800-850. Attaching a good diagnostic tach showed me the error and I can set idle ok where I want it.

Engine feels rough at 850. So replaced plugs, checked timing, rebuilt carb and ran it pulling wakeboarders for a day. No problems except that rough idle. So hooked up a vacuum gauge. And I am stumped. I have never seen a vacuum gauge work like that in an unblown engine. I could see it with a high lift cam, but I can't believe anyone would make one for a 3.0L, let alone install one. At idle to about 1300 RPM the needle rapidly oscillates between 10 in and 15 in Hg. At 2000 rpm it settles down to 17 inches with barely a flutter.

Maybe the engine was rebuilt and they did the valve guides incorrectly?

I am going to try and post a link for a video I shot of the vacuum gauge. I have had a lot of trouble posting links, so I may not get it done. We'll see.

Youtube video https://www.youtube.com/watch?v=w_boOaxU8FM

Rick

Could be a few things and not always valve guides. Could be uneven compression (broken rings or leaking valves or head gasket in one or two cylinders) is a likely culprit. An uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes. What are the compression numbers?

 

[BF]

Hiya,

I've got a 1990 3.0L in a SeaRay 160... I watched your vid, it seems to me that it might just be the needle jumping around b/c of excessive engine vibration. Does it act the same if you've got the gauge in your hand? I just went through diagnosing and fixing a very rough idle and hesitation in mine. Was a bad coil. One thing I forgot to mention in that thread is that the tach sometimes read too high (like yours does), and the timing light appeared to be misfiring a fair amount when I checked my timing. Since I've replaced the coil, the tach reads correctly. So i do think the ignition system was behaving badly and cause a very rough idle with missing, and big engine vibration. I'd expect that caused it to idle too rich and which might've contributed to the bog out of the hole (?).

Here's the link to my thread.... maybe if you went through the DDIS diagnostics, you'd turn up something. (I'm assuming we have the same 1990 ignition system, but maybe yours has been changed out).

http://forums.iboats.com/forum/engi...al-distributorless-ignition-system-ddis-probs

Good luck.

 

[Rick Stephens]

Thanks for the replies. Never even considered ignition at all - interesting. Compression was virtually identical in all cylinders at 117. If I hand hold the gauge it reads the same.

 

[Rick Stephens]

I believe I can prove/disprove the ignition system being possibly the cause just by running the vacuum test with the ignition disabled while cranking with the starter.

 

[Fastatv]

I believe I can prove/disprove the ignition system being possibly the cause just by running the vacuum test with the ignition disabled while cranking with the starter.

Have you checked for vacuum leaks...guess you have and I may have missed that when I read your post. Did you adjust the air/fuel screw for the highest, cleanest idle? Here's a very very long shot, and I have never actually seen it before. Idle is rough at 850, settles down at 2000 rpm.....is there any possible way that maybe its not charging well at 850, and your source voltage is low...causing a miss at the low rpm? I know, really sounds out there LOL but, sometimes the issue is "out of the box".

 

[Rick Stephens]

Spent a week going through things. Did a running valve adjustment. Nothing there. The internals on the engine are so clean as to be like new. Not a spec of carbon build up and absolutely no oil usage after an entire season. Well, none until I did a running valve adjustment

I have tried everything I can think of to find a vacuum leak short of blocking off everything and trying a pressure test. Spraying carb cleaner all around the intake/exh gasket induces no reaction in engine rpms at all.

When I did the carb rebuild I adjusted the idle mixture screw with a tach. I have redone that several times after making other changes to make sure it is still correct.

Timing is spot on.

Compression test virtually identical 115 PSI for all 4 holes.

New AC Delco plugs matching recommended.

Battery charge voltage is up over 12.8 volts.

Vacuum gauge settles down around 1200-1250 rpm to 17 inches Hg. Still rapidly oscillates between 10 and 15 inches when under 1200 RPM.


I haven't been able to make a change in anything yet. Engine has lots of power, uses no oil, runs smooth under load and at speed, and idles crappy. I can't leave it be - it's supposed to be smooth and silky at idle or I haven't done my job right.

Is there a way to test valve guides? The only diagnosis I ever see for vacuum gauge oscillation is valve guides. That won't show up in a leak down test though,

Thanks for the help!

Rick

 

[alldodge]

Spent a week going through things. Did a running valve adjustment. Nothing there. The internals on the engine are so clean as to be like new. Not a spec of carbon build up and absolutely no oil usage after an entire season. Well, none until I did a running valve adjustment

I have tried everything I can think of to find a vacuum leak short of blocking off everything and trying a pressure test. Spraying carb cleaner all around the intake/exh gasket induces no reaction in engine rpms at all.

When I did the carb rebuild I adjusted the idle mixture screw with a tach. I have redone that several times after making other changes to make sure it is still correct.

Timing is spot on.

Compression test virtually identical 115 PSI for all 4 holes.

New AC Delco plugs matching recommended.

Battery charge voltage is up over 12.8 volts.

Vacuum gauge settles down around 1200-1250 rpm to 17 inches Hg. Still rapidly oscillates between 10 and 15 inches when under 1200 RPM.


I haven't been able to make a change in anything yet. Engine has lots of power, uses no oil, runs smooth under load and at speed, and idles crappy. I can't leave it be - it's supposed to be smooth and silky at idle or I haven't done my job right.

Is there a way to test valve guides? The only diagnosis I ever see for vacuum gauge oscillation is valve guides. That won't show up in a leak down test though,

Thanks for the help!

Rick

Yes there is a wiggle test, see video link below. Keep in mind the production clearance is:
Intake .0010 - .0027
Exhaust .0007 .0027

Service range is
Intake .03 Max
Exhaust .04 Max

https://www.youtube.com/watch?v=vObzaHECEos

IMO you don't have an issue which needs to be fixed, it will last a long time as-is.

 

[Rick Stephens]

Yes there is a wiggle test, see video link below. Keep in mind the production clearance is:
Intake .0010 - .0027
Exhaust .0007 .0027

Service range is
Intake .03 Max
Exhaust .04 Max

https://www.youtube.com/watch?v=vObzaHECEos

IMO you don't have an issue which needs to be fixed, it will last a long time as-is.

Thank you. I'm going to run out and pull some wakeboarders around manana. See if my tweaks make any difference. I expect you are correct and I am splitting hairs. And part of the idle issue is just the nature of a 4 banger. Heck, part of the vacuum gauge may just be 4 banger.

Rick

 

[Rick Stephens]

This motor doesn't idle low enough. 8-900 is as low as it goes without getting too rough to keep running when shifting. So decided I would pull the head and have it redone where needed.


Before doing that I ran the compression tests again and added squirting oil in.

Pressures pretty much the same from cylinder to cylinder: 115 PSI without oil and 150 PSI with 3 squirts of 30 wgt.

Oil pressure according to the guage is around 38 PSI, pretty much all the time.


Question is, is that enough to warrant a rebuild? I'll have the motor apart, so it isn't that big a deal. And be nice to rid myself of the lil' oil seal leak. But pulling the pistons ups the scale and starts piling on the money, at least compared to just doing valves.


Thanks for the thoughts.

Rick

 

[Silvertip]

If you are trying to reduce the idle speed without changing the idle mixture with each attempt, then you are chasing your tail. Idle mixture is not one size fits all. Lower the idle to a point as low as it will go. Adjust the mixture. Reduce the idle speed some more and repeat the mixture adjustment. Repeat the process until you are at the recommended idle speed. If valve guides were so bad as to cause an idle problem, the engine would belch blue smoke at startup because oil would go down the valve guides.

 

[alldodge]

This motor doesn't idle low enough. 8-900 is as low as it goes without getting too rough to keep running when shifting. So decided I would pull the head and have it redone where needed.


Before doing that I ran the compression tests again and added squirting oil in.

Pressures pretty much the same from cylinder to cylinder: 115 PSI without oil and 150 PSI with 3 squirts of 30 wgt.

Oil pressure according to the guage is around 38 PSI, pretty much all the time.


Question is, is that enough to warrant a rebuild? I'll have the motor apart, so it isn't that big a deal. And be nice to rid myself of the lil' oil seal leak. But pulling the pistons ups the scale and starts piling on the money, at least compared to just doing valves.


Thanks for the thoughts.

Rick

I would say if you can deal with the rough idle (sounds like you cannot) run the engine until it quits. Now if you want to fix it and get it smoothed out then a rebuild is in order. You gained 35psi with oil which tells me your rings are worn. You also said it has plenty of power so the money decision is yours. I would hate a rough idle and would do the rebuild, but that's me. Sounds like you really don't need to and could save the coin

 

[Rick Stephens]

Silvertip, I always do readjusts on idle mix with a tach. Even sitting in the driveway on muffs I can't get the idle down where it should be. I've never owned a 4 banger before, so some of the questions are not knowing what to expect when doing diagnostics.

AllDodge kind of nails it. I had to ask though, the pressure gain with oil was more than I expected.

The boat is the right one for me moving forward. And hard to just sit on it running like a 3 legged mongrel, hoping when it does puke something I am close to home. The motor's probably original with enough hours to benefit from a rebuild. And it's not like I didn't pick a 3.0 Liter 'cause how easy to work on. Big part of my choice. So I guess I quit hemmin and hawin and dig in, or shutup.

Thanks.

Rick