Cobaltchat.com

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)?

.
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.
.

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.

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

Do not adjust throttle cable to compensate.

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.

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.

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?

Craig O wrote: ↑Thu Oct 05, 2023 7:49 am 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.

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.