Bogging down and stalling ONLY at idle

[skydiveD30571]

volvo penta 5.0 gl-h

It was idling a little high the first time out a few weeks ago (about 800-900rpm), but otherwise ran great. Took it out yesterday after i did cap/rotor/plugs/wires, tweaked idle speed screw, and replaced a leaking exhaust boot. It now has an intermittent idle issue.

On the driveway with the hose it choke started great and idled steady at 600rpm for 10-15minutes. I thought, "awesome, we got the idle speed just right." Then rpm's started bogging down and returning to 600, like it was starving for fuel. In a hurry to leave, I shut it down and trailered it out.

Once in the water, at first it idled fine, but after half an hour or so it usually would bog down and fluctuate from 200rpm to 600rpm. This seemed more common after a load was put on (i.e. pulling a skier). Throttling back to idle it would immediately bog down to 200rpm then back up to 600, taking about 1 second per iteration. It was a constant rythym when doing this. Many times at the low rpm side it would stall out. By the end of the day, it needed throttle everytime it started to continue running, and would usually stall if left in idle for more than a few seconds.

It is hard to explain because it was so hit-and-miss throughout the day, but it definitely got worse as the day went on. So the first 30 minutes of the day it was basically fine, and the last hour or so it consistently bogged down and stalled only at idle.

Also, once out of idle, it runs great all the way thru wot. It is definitely only at idle (both neutral and in gear). I tried searching for forums, but most people with a bogging issue have an engine that idles great then bogs down once given throttle. Mine is the opposite.

At first I was thinking a fuel issue on the boat side, but I got to thinking, wouldn't that affect the whole rpm range not just idle? Being that during choke start it idled great, it seems almost like a lean idle once the choke is fully open. Any ideas? I don't think a carb rebuild would hurt, but I'd like to pinpoint the issue to the carb before forking out the money or attempting it on my own.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Since I did that tune-up stuff over 2 different days, and took the boat to the lake on the day inbetween which is where the idle issue originally started, I'm trying to remember what I did the first day.

New plugs. Called o'reilly's to see if they carried the AC Delco plugs that I pulled out (original plugs). They confirmed. When I got there, I asked about NGK plugs and he said they have those too and recommended them. He gave me the G-Power version in a yellow box, which I gapped to volvo spec.

Carb cleaner. I let the engine warm up on the hose and used a can of carb cleaner to spray around the carb body because there was black residue buildup on the choke plate. Soon afterwards is the earliest I can remember the idle starting to fluctuate, but it was surging not bogging.

Adjust idle speed screw down a little bit. That next time out, it started up and idled right at 600rpm like intended.

Could this weird idle behavior be caused by spark? I can't imagine how new spark plugs that were gapped to spec could cause an issue, especially only at idle. Maybe they are the wrong version? I did read somewhere that a bad spark can cause erratic idling. Then again, maybe that carb cleaner loosened up a bunch of junk in the carb which has since blocked the idle jet. Any ideas where to start would help a ton. Thanks.

 

[Don S]

Re: Bogging down and stalling ONLY at idle

Remove the fuel filter and pour it's contents into a clear container and see if there is a lot of water in it.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Never reuse filters right? I just ordered a new one. Once I remove the current one I won't be able to run the engine until the new one comes in, so is there anything I can/should check before I do that last?

 

[Don S]

Re: Bogging down and stalling ONLY at idle

Yes, no reusing filters.

here's the thing. Sitting in your driveway, and the outdrive down, I would bet the front of the boat is lower than the stern. Which means any water in the tank would go forward. So, when you run the engine on land, all is fine. When you put it in the water, all the water in the tank moves aft to the pickup. The boat will run fine for a while, until the fuel in the filter is replaced with water, then you have water in the carb. Usually it will idle fine, as there is enough gas to let it run a little bit, but when the demand for gas goes up, there isn't enough there for it to run.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Before I removed the filter I figured I'd go buy a vacuum gauge and see if it sheds any more light. Hooked it up to the pcv value and let it warm up. All seemed well so I shut it off, looked around, and started it back up again. Right off the bat it was bogging way down, and of course the gauge was all over the place. I began playing with the throttle to figure out what the deal was. Now I didn't get much time because a storm rolled in and I was trying to beat it, but here is what I saw:

I pulled on the throttle plate and revved it up to keep it from bogging down and dying, but when I let go, it dropped to about 1000rpm and stayed there. I grabbed the throttle plate and pulled on it and sure enough, it bogs down and almost dies. I then adjusted the throttle while moving the idle speed screw in and out until forcing the throttle closed resulted in the idle speed I wanted. I then revved it up and let go, and sure enough it didn't come down and stop on the idle speed screw. Don I know you recommend adjusting the cable to give a slight closing force, which I will do when the storm passes.

I think that explains why, on the water monday, gradually letting off the throttle sometimes kept it from bogging down, but quickly returning to idle almost always killed it.

At this new idle speed, the gauge sat consistently around 17in, with a small flutter almost up to 18. So a pretty small change. I didn't rev it too much since its out of the water, but upon revving it would drop then shoot up like the service manual says it should.

I set the idle speed just a couple weeks ago, so I'm wondering if the spring is somehow weak or loose enough that the screw is vibrating to a new setting. How much force should the idle speed screw spring put on the screw? It is pretty easy to turn.

It purred pretty good for the next 5 minutes with a nice consistent smooth idle. I walked off for a few minutes and saw idle speed closing in on 900rpm. I came back and forced the throttle closed and it returned to normal. So I guess my question is this, does it sound like there is actually a very small pull on the throttle cable to open it instead of pushing to close it? I haven't heard of that before. I'll also be checking the fuel filter since the new one is on its way.

One last question, I had the flame arrestor off while doing this, so the valve cover return hose (the red arrow in the attached picture) was open. It had a very small amount of smoke coming thru it. Does this always happen but I never notice because it gets sucked down into the carb when the hose is attached? Thanks again.

View attachment 149547

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

Tha smoke/vapor is normal, and as you suspect it happens and is not noticed as it just gets drawn into the flame arrestor. If an engine is in very aged condition then that venting can become excessive but you would have other symptoms if it was an issue.

Regarding the other symptoms, some of them sound like a vacuum leak, those can cause the up and down looping on idle and higher idle. A clogged flame arrestor can be a factor as well, the engine will "find" other places to draw air in if the arrestor is clogged. The rpm's will climb until the mix leans out then drop back down, the mix catches back up at the lower rpm's and it climbs again, etc. Spraying carb cleaner in spurts around the base while it is running is one method to see if the base gasket or other carb part like throttle plate bushings are leaking air, if the engine changes how it runs when sprayed then that part of the carb may have a problem.

I also see that you have physical "re-opening" of the throttle plates. The throttle movement from the controller should not be a factor there as going into gear from neutral does not cause throttle movement immediately but happens later in the lever travel. I may have missed where you mentioned a return spring issue, or a cable that sticks.

I am not discounting water in the fuel being a factor, that pool of water will move around a lot in the tank and also in the carb bowl, much like the bubble in a level.

I can't remember if your carb is a 4bbl, but if it is and it is a Holley it is interesting to note that restrictions in the primary flow (clogged arrestor) can cause the secondaries to want to open and if there is a light spring in the secondary actuator vacuum pod then the rear throttle plates can actually open slightly causing higher idle.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

I suspected a vacuum leak too which is why I bought a gauge to test it out. Once I got the idle where I wanted, the vacuum gauge was showing consistently 17-17.5" which according to the service manual shows a properly running engine. I just need to adjust the throttle cable to push the throttle closed since it wont on its own power. Do those idle speed screws ever have problems with moving around? Seems like the spring would take care of that, but maybe its worn out.

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

I will be (working) on my boat this weekend, I will look over my carb linkages to see what I can see.

 

[insttech1]

Re: Bogging down and stalling ONLY at idle

I had a similar problem on my boat last year and when I got her running this year. I got thru the last season by running idle at probably 800rpm, and this year I got sick and tired of it. I completely took the boat throttle cable off, and concentrated on the carb itself. (The carb had been freshly and thoroughly rebuilt.) My problem was a combo of four adjustments: Idle screw not reacting properly (wouldn't go low enough RPM), tang/pawl to the diaphram pump was not bent correctly, and the same tang was also partially contacting the main metal throttle assembly, only at the point where the carb/engine comes to idle, instead of simply riding on the orange plastic cam that it's supposed to. And, the spring pressure to the diaphram pump was set to factory spec' with a feeler gauge, but it had to be tweaked a bit as well after bending the related pawl (see below). So the final resolution was to tweak the idle adjustment screw all the way out until there was no contact, then screw it in until I had at least 100 rpm more than what the engine was running at (so I knew I had it in a good adjustment range). Then I had to experiment with bending the tang going to the diaphram, and increasing spring pressure on the diaphgram to lower RPM to the intended range. After those two adjustment were made, then I used the main idle screw to actually set the proper idle (because now it is in a range where it actually does make RPM go up OR down). After all that was done, and the throttle was returning to a consistent place, without binding, whether it was pushed quickly or backed down slowly, did I re-attach the boat throttle line and set a little preload. Giant pain in the ... but I now have a consistent and adjustable throttle.

I guess the previous owner had just decided that living with it that way was fine. It drove me nuts. I think they did this because the carb wasn't tuned for crap...the float was way out of adjustment, as were the idle mix screws. They also had the throttle cable adjusted about nearly 3/4" past the end of the throttle attachment. (Past the closed position.) So I had to to tweak that too until there's just light pressure against the throttle when the hand control is set to neutral.)

Getting the correct Holley manual also helps to know how/where to actually bend the carb parts that don't have adjustment screws.

You said you checked vacuum, but did you actually pull vac' from the carb and adjust any individual idle mix screws (not the main idle RPM screw?)

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

I did not adjust anything else, including the idle mixture screws. It idles smooth and consistent when the throttle cable actually lets the linkage return to the stop, and the speed screw is adjusted correctly. I'll see if adjusting the cable to provide light pressure against the throttle does the trick on the water, and if not I'll begin looking at the carb tuning and adjustments. I do feel like this resolves the issue based on how it was running afterwards.

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

Skydive, I buttoned mine back up before I took a pic, but the design of my cable would not be able to put forward pressure on the throttle lever without some kind of hack. The throttle lever on the carb gets pulled back by the cable end that is in front of the lever. There is one large return spring on the bottom of the throttle lever on the carb that goes back to the throttle cable bracket at the back of the intake manifold.

I may have the engine cover off tomorrow and if I do I will try to take a pic or two.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Took it out last night for a test run...with good and bad news. The good news is it idled great in gear, nice and smooth right at 600rpm. In neutral it idled slightly higher at about 800rpm. Is that too high? And is there a way to adjust neutral idle without affecting in gear idle speed? Does this engine/drive combo use a shift interrupt mechanism? The reason I ask is because of what happened on the way back to the dock. I started a new thread because it is a new problem.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

I did some investigating and tried to find a vacuum leak yesterday while on the hose. The first picture shows the pcv value I pulled from the valve cover.



I took the tube from the vacuum gauge and stuck it in the hole in the pcv value to find vacuum (picture 2).



Should vacuum or idle change when the pcv value is pulled from the valve cover? If I put my thumb over the end of the pcv valve to plug it, the boat idles down to exactly where it should be. I'm not sure if this is normal of a pcv valve. Also, when the valve is inserted in the valve cover like usual, I can feel the little ball/plate in the pcv valve bouncing around. Is it supposed to be steady? There is also a small whistling sound coming from that area. Does this sound like the pcv valve could be bad, maybe not closing like it should during idle, or not seated correctly resulting in a loss of vacuum and causing the high idle? There was also fluctuation in the vacuum gauge up to 1" but if plugging it into the pcv valve is incorrect then this may not be an accurate reading.

I sprayed around the carb and intake manifold with carb cleaner and could not find a leak anywhere.

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

The following is how I understand the carbed VP's "PCV" plumbing, I could be mistaken about your particular model.

The PCV should have no valving guts in it at all. The PCV is used just as a fitting, and the hoses run to the flame arrestor and not to the carb base. There should be one PCV on each valve cover with the hoses running to the flame arrestor, one on each side.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Maclin I have seen exploded views of models that you describe, but mine is different. The port side valve cover has a breather tube coming from it and leading to the flame arrestor. The starboard side valve cover has what the manual says is a pcv valve and is leading to the carb base. This hose does have suction, but the breather tube has no suction at all (obviously).

Exploded views I have seen indicate grommets around parts of the pcv valve. As you can see in the picture I attached above, mine has no rubber grommet, just a metal cylinder that slides into a hole in the valve cover. Maybe this is where the low-volume whistling noise is coming from? I may go buy another pcv value since they are pretty cheap. Just another possibility to eliminate.

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

The ball inside is to block any positive air pulses, like backfires, from going into the crankcase from the intake manifold. I think they bounce around, they get pulled off their seat just by engine vacuum. If you can hear whistling noises then that is a source of vacuum leak I would think.

I was not sure how new your engine was, I thought it was "before" the EPA caught up with boating

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

The engine has a mfg date of 9/2006, so definitely after EPA's involvement. I was reading up on vacuum leaks last night, and some people suggest pinching the pcv hose and if idle drops more than 100rpm or so, its time to change the valve. Apparently that shows that it is allowing too much air thru at idle speeds. I imagine plugging the end of the valve with my finger is essentially the same concept, and it drops 200-250rpm when I do that.

 

[skydiveD30571]

Re: Bogging down and stalling ONLY at idle

Any ideas on what would cause a high idle in neutral but not in gear? I'm afraid that getting the neutral idle down to where it should be will cause the in-gear idle to be too low. Is there a possibility that a vacuum leak would be apparent in neutral but not when the engine has a load on it?

 

[Maclin]

Re: Bogging down and stalling ONLY at idle

A high idle in neutral due to a vacuum leak will usually cause the engine to die when placed in gear, especially if the idle adjust screw was turned down to alleviate the high idle in neutral as you mentioned.

I would be tempted, just for diagnostics, to plug the carb PCV inlet and run it like that, trying neutral, in gear, idling thru the water, etc.