97 LK 5.7 backfire on rapid throttle from idle

[nathanj988]

I am still working my way through my new boat before I try it on the water. I just replaced the thermostat because it was missing one and wouldn't get past 110. I let the engine warm while on the muffs and then once it was up to temperature after a couple minutes, I noticed it wasn't idling so well like it was missing. The engine doesn't want to come off idle and if I punch it, it backfires and I see a puff of gray smoke from the engine cover.

When I bring the idle up to 1000-1500, the engine smooths out. When trying to slowly bring the idle up, the engine takes on a sucking sound that reminds me of a car engine when you pull a vacuum line and it struggles to run. If I continue to throttle up, it gets past that bad area an revs just fine.

I plan on checking the vacuum but there doesn't seem to be a vacuum port. Intake port 8 has a plug that I'll remove for my vacuum source, hopefully that will come out cleanly. I don't remember it idling like this or having a problem punching it before I replaced the thermostat. This may be the reason it was missing the thermostat, to prevent it from warming up and running rough.

Any ideas?
Nathan

Edit, I forgot to mention this a TBI setup.

 

[Bondo]

Re: 97 LK 5.7 backfire on rapid throttle from idle

Ayuh,.... Check the Contents of the fuel Filter,.... sounds like it's goin' lean,...

 

[nathanj988]

Re: 97 LK 5.7 backfire on rapid throttle from idle

It is possible. According to the previous owner, it hasn't been in the water since late 2011. The first thing I did was extract some fuel though the fuel line and it looked good and smelled good. It also burned well. It has a 1/4 tank of gas.

I do have a spare filter. I wouldn't expect it to idle poorly though because that is the least demanding for fuel flow. It also sounds good when revved pas 1000-1500, although it probably needs very little fuel while in neutral.

I need to try revving it with a cold engine to see if it only misbehaves when warm.

 

[nathanj988]

Re: 97 LK 5.7 backfire on rapid throttle from idle

I tested the vacuum and it is low but steady. I got the same reading on the cold engine and when it warmed up, 14.

The idle was the same on the cold engine as when it warmed up , also.

I was able to punch the throttle on the cold engine and it responded quickly, however I could make it backfire with a very fast throttle punch.

Once the engine got to about 120 degrees, it would run with low vacuum and sometimes almost cut out after allowing the RPM to come back to and idle. When it as behaving like this, it sounded hissy, like a car when you pull a vacuum line. It would run with this low vacuum or 5 to 10 seconds and then recover to a normal idle.

I guess I'll check timing next.

 

[nathanj988]

Re: 97 LK 5.7 backfire on rapid throttle from idle

I set the base timing to 8 BTDC using a Craftsman adjustable timing light and the vacuum came up to 15" from 14". Even if I advanced the timing quite a lot, it still wouldn't come up to 20". I need to make a mark on the harmonic balancer at 8 degrees so that I am not relying on an adjustable timing light, I don't trust them as much as a simple timing light.

Something was done to this engine in 2010 before the previous owner had it, and I'm not sure she would have known if everything was right or wrong. Anyway, I think I may want to check the valve timing because that is the other reason for a low vacuum.

Is there an easy way to check valve timing? I think to remove the timing chain cover I need to remove the oil pan. At that point it seems like I would just pull the engine from the boat.

 

[nathanj988]

Re: 97 LK 5.7 backfire on rapid throttle from idle

I warmed the engine up and then did a compression test. All but 1 were above 170 and the other was 166. The highest was 173 so it passed that test.

What could be the reason for my lowish vacuum? If my vacuum is 16 at idle could my valve timing be off or would my compression not be so high?