97 XR6 Intermittent loss of MPH

[DavidDC]

While making a run on my local lake I had a sudden loss of about 5-7 mph at WOT. It lasted about 5 seconds an then went back to normal MPH. This happened several times on that run and then stayed at the lower MPH until I shut her down and went home. I changed the spark plugs and went out the next day with no issues but the day after that I had the loss about 3 miles in my run and it never came back to normal MPH. At Idle it sounds different. Kinda "muffled" but idling fine not missing or idling rough. I don't want to do any damage but the water is calling my name. Thoughts?

David

[EuropeanAM]

Need to get this checked in a methodical, deductive-reasoning approach/method:

-Compression Test
-Spark Test
-Fuel Quality & Delivery
-Inspect Exhaust Tuner (drop gearcase, long screwdriver, make sure firmly secured to adapter plate)

I would not recommend you continue running it, as any FUEL DELIVERY problem is ALSO AN OIL DELIVERY PROBLEM. That can get costly in a big hurry if you continue running it.

BTW... next forum down has lots of information for 2.5L engine models (such as yours).

[OldTimer57]

I had an XR6 that did that years ago. Turned out to be a misbehaving ignition coil. It would work until the motor was put under load. Then it was iffy and losing 1 cylinder is a big deal. I finally caught it running on muffs. Suddenly started idling ragged (more ragged than an XR6 idles since they always idle pretty roughly compared to the DFI motors). I ran over and started yanking plug wires and found one that made no difference. Replaced the coil and all was well.

But as Don mentioned, you need to have a bit of "engine feel" experience to determine whether it is ignition or fuel. Fuel is bad news for the reason he gave. Misbehaving as you describe suggests ignition woes to me.

There are other possible causes, ie a bad plug wire, a bad spark plug. I once swore my motor was running lean and had a fuel problem in it would run just a bit at WOT and then start to sag. I cleaned everything. Finally started replacing plugs and when I hit one plug, it was back to normal. I could not see a crack in the ceramic insulator, but with surface gap plugs they are not always visible.

[DavidDC]

I finally was able to do a spark test and one of the cylinders is not getting a spark. What should be my next step before I take it to a mechanic?

David

[EuropeanAM]

Did you use a timing light or a spark tester?

Engine serial number?

[JOE54]

well you can swap green wires on coils to see if no spark follows wire swapped, if so likely switchbox bad if not and same coil has no spark coil likely bad make sure plug is good if tested with timming light, good luck try v6 section one down for more info on yo mtr

[DavidDC]

I swapped coils, spark plugs and plug wires out with good ones from the other side, still no spark using the spark tester. I'm thinking its the switchbox. Any way to test it?

[DavidDC]

Spark tester. Where would I find the serial number?

[EuropeanAM]

The "low cost" method to test the switch box is to swap switch boxes (inner and outer). Be very careful- grounds are very important (case grounds), and don't intermix the leads from the inner and the outer boxes.

Note: Switchboxes should be replaced IN PAIRS (the faulty switchbox often damages it's mate's Bias Circuitry).

[leonreno]

Serial number is usually on the transom mount on starboard side.

[SilverFox579]

Serial number is usually on the transom mount on starboard side.

Or a welch plug on the block. The second sticky at the top of this forum has pictures - How to find your serial number on your Mercury outboard

[DavidDC]

Any help with the switchbox part number? My motor serial #06509171

[JOE54]

7778A12

[DavidDC]

I swapped the switch boxes and my loss of spark moved from the bottom port side cylinder to the bottom starboard side cylinder so it looks like I'm buying new switch boxes. Ouch! They are proud of those little fellas! $$$$$$$

[JOE54]

you can go cheaper just not always better though