Engine surges when warm
 

Working on a 1995 7.4L 330HP engine with Thunderbolt V ignition. Here is what I have replaced, Cap, Rotor, pickup coil, Coil, plugs. Only thing left is the ignition modual ??? Engine runs smooth, does not idle rough and does not have a miss. I can pull full RPM at times. Seems to vary by about 300 RPMs when it surges. I would just replace the modual but they want a huge price for it. 806959 is the part number.

how olds the gas and when was the fuel filter changed last?

Good question, but defiantly not fuel related. New fuel filter, rebuilt the carb and fresh gas.

I am working on this boat for my brother. If I knew what I know now I would have dropped in a DUI and called it good.

Does it surge at idle ? Or a particular rpm when under load ? Or all rpms under load ?

Pull the distributor and see how much end play there is. I bet a lot. Or take off the cap and crank the motor over and watch the rotor jump up and down. The gear on the distributor is probably toast as well.

put a timing light on it and see if the timing is moving with the surge

He'll have to put it in 'base mode' or what ever it's called. He has the Tbolt V, which constantly moves the timing around for what they call 'mean best timing.'

Not as noticeable at idle. More prominent under load, does it at all RPM. I have not water tested with new plugs yet but they looked good.

Again this points me toward the ignition modual.

That may be my next step. I thought I had the problem fixed with the pick up sensor. Did not have the timing light with me.

i,m not saying it can,t be an ignition problem but surging is almost always caused by a fuel problem.

A lot of time surging can be fuel related. With that said it has to be a process of limitation. And of course timing / or ign related related to. Ign related is more of a miss and can get worse.

I would put a fuel pressure gauge on this first and make sure that is not a problem. (fuel delivery problem) Next I would run a true intake vacuum source test with a Vacuum gauge. Vac testing can tell you alot.

I would also run the engine on a new portable fuel supply with fresh gas and new fuel line with a primer bulb inline. Squeeze the bulb if the surging is still taking place . Anything change

A layman's simple test also is while surging spray some carb cleaner down the carb did the rpms pick up or did the engine want to stall or did your surging go away ? very basic test but can pin point a fuel problem or a very lean condition.

Merc has a T bolt ignition flow chart for testing the components that can help as well.

Looks like the Thunderbolt V has modual and knock sensor. This is most likely where the problem is.

Ordered the knock sensor today. I will report back on the results.

Make sure you use the correct torque value.

What made you determine the knock sensor is the issue ?

If he has the OEM service manual or google it they Merc have a testing procedure for knock sensor and knock module right after for the T bolt V ignition testing flow chart.

Knock sensor install is 12 to 16 ft lbs no more and nor less installed per Merc specs. I just did one on a 350 HO mag.

If the knock sensor is bad it can retard the timing dropping about 300 RPM. And it is the cheapest part to try next.

Thanks for the tip on the torque value.

What symptoms did you have on the one you replaced?

lagging and seemed not crisp throttle response

Knock sensor fixed the problem.