7.4 Gi Dies shifting into gear

[Craig O]

I have a 1997 7.4L Gi, DP-S drive, that will die when I shift into forward or reverse gear.

This only happens when the engine is up to operating temperature and idling at 600 - 614RPM (digital tach). When the engine is cold, idling at 650 RPM it has no issue going in and out of gear. I tried to remove the rear prop and it has no problem at operating temperature spinning 1 prop.

There are no engine faults detected, the fuel pumps were replaced a year ago, along with plugs, wires, distributor, and IAC. For kicks I pulled the vacuum line of the fuel pressure regulator, the extra air sucked into the intake with the line off bumped the idle RPM up to 650, and the engine had no trouble shifting into gear.

Is it acceptable to adjust the throttle cable to force the Idle up to 650 RPM even though the ECM is trying to control it to 600? Is there something that I should be looking at to explain the somewhat weak idle when its at operating temperature (>130F water temp)?

[AsLan7]

.
So glad you found us Craig O.

Those sound like symptoms of a bad idle air control valve (IAC). Hard to diagnose over the internet though.

Others will chime in shortly.

And welcome aboard mate.
.

[Craig O]

Thanks for the welcome!

The IAC was replaced about a year ago, and based on the checks in the service manual it appears to be operating correctly. The book suggests to run the engine, shut it down, then disconnect the IAC. Fire it up disconnected, verify it's idling At least 200 rpm higher, then reconnect and ensure it drops back to 600rpm. The IAC had no issues with this test.

I'm open to any suggestions.

[bruceb58]

Have you changed spark pugs, cap lately? Fresh fuel?

Do not adjust throttle cable to compensate.

[Craig O]

Should be good on plugs, cap, and fuel.

I understand adjusting the cable isn't preferred, it's just 40 RPM from making it and it's really tempting.

Interested in hearing any other suggestions. The boat seems to run great otherwise. Tops out at 4,800 RPM on the high side, fires right up. No service engine codes.

I'm a little bit at a loss right now.

[bruceb58]

It's definitely odd especially since you replaced the IAC already which is what should be keeping this from happening. What was the reason you replaced the IAC a year ago...same issue?

Ever thought about getting your injectors cleaned?

[Craig O]

I just bought this boat, assumed this would something easy enough to tackle myself.

The service records I have show that the complaint was hesitation when accelerating from idle.

The shop replaced plugs, wires, cap, distributor, and noted the IAC was not functioning and replaced it too. 6 months prior they put new fuel pumps in as well.

I thought about injectors but the engine seems to run really strong in all conditions except this scenario. It's strange that with a cold start up this issue isn't there. My thought was the injectors would be a more consistent issue and not temperature dependent.

Do you have a recommendation for getting injectors cleaned if that ends up being the issue?

[bruceb58]

Do you have a recommendation for getting injectors cleaned if that ends up being the issue?

Just google it. There are a bunch out there. Typical cost is around $20/injector plus shipping.

I had low speed and idle issues with a vehicle I owned that had a little less than 200K miles on it. I just replaced the injectors and it made the vehicle run like new again.

[NautiWeasel]

Craig O, did you ever get this figured out? It sounds like a fuel issue to me (too lean) When engine is cold, it is going to choke the engine, giving it less air (more fuel) and RPM will jump a bit. It's already higher on the RPM band, and will be quicker to respond when given more throttle. I don't know that engine, and assume it's new enough that a computer is controlling fuel delivery, so maybe look for a vacuum leak somewhere. Could even been a brittle hose causing a vacuum leak, which in turn would be a lean situation. The IAC and computer can only do soo much to clean that up, so if you have a leak, it could be causing your issue.

[Craig O]

I am still trying to find a solution. I found some information on the MAP sensor being a potential issue since it is responsible for sensing a change in load, but a new sensor had no effect. I am considering replacing the IAC to see if the one that went in a couple years ago was somewhat defective but I'm really just throwing parts at it with no real direction.

The local Volvo Penta shop I talked to seemed to think bumping the idle speed up was the way to go so I really don't feel confident using them as a resource.

This engine is under 400 hours, clean as can be, fresh oil, fresh gas, has no other symptoms. The only other related issue is the shifting into gear seems a little harsh, but I am used to running a single prop bravo drive. I don't have a feel for what a dual prop should feel like, it may be great or may be too harsh.

I appreciate the suggestion on the vacuum leak, I may look into this a little further.

This boat also came with some delamination on the gel coat that proved to be much more expensive to repair than expected, in combination with the reliability (dies going into gear) making docking and trailering on a river a nightmare I am about ready to throw in the towel.

[jhnmdahl]

On a '97, you may have fresh gas but fouled injectors. Quick fixes like seafoam may help, but cleaning injectors isn't a terribly expensive thing to try (or to have done anyway after a quarter century).

Thanks for keeping us posted,

John

[Craig O]

I gave in and took it to the nearest authorized Volvo Penta shop and they came back saying its a real head scratcher. I pointed to the drive and asked about that and they said it all checked out good. The ran it on the dyno and it actually has maybe 5 - 10% more power than the spec. To them with the scan tool everything looked great. They noticed the water temp ran low, about 140F, and mentioned most run 160F. They said the idle may be slightly rough and may not hurt to clean the injectors, but the ECM is where they would go next.

I found in the service manual that the engine is supposed to run a thermostat with 140F opening, so no issues there. I went ahead and pulled the injectors and had them cleaned and flow tested, no issues there, and no change to performance. I feel like I am left with the ECM, but I have no logical explanation of what may be happening other than maybe a delayed response for some reason. Maybe the driver for the IAC is not providing proper voltage or drive current and the stepper motor is intermittently hanging up?