1988 XR4 - Faulty Fuel pump symptoms?

[gaventure]

I have a 1988 Merc XR4 on a 1988 Venture Bass boat that has been great until a couple weeks ago.

I was out taking picures and marking lake structure with my GPS. I had been on the water for several hours without a problem. As I was about to leave my last stop, the engine sounded "flat" as I give it throttle. It did plane out, but I could tell it was lacking a lot of power. I was only a 1/4 mile from the ramp, so I took it back and put new plugs in. I thought one or two may have fouled due to idling around more than normal. No deal. Plugs didn't work, so I headed home to troubleshoot.

• Compression on all cylinders is around 140.
• I have fire on all cylinders, although when I took the wire off of #1 and #6, there wasn't any noticeable difference in the way it ran. Especially on #1.
• I took the carb off of #1 & #2 to clean. No visible problems. It was extremely clean.
• Put it back together and it ran the same.
• I had an extra set of Switch boxes, so I swapped them out to check. Nothing, so I put the originals back on.
• I had an extra set of coils as well, so I replaced the coils on #1 and #6. No deal.
• I then turned to the fuel. Took a sample and let it sit overnight. Water was visible in the bottom of my sample.
• I drained the carbs, removed, drained & dried the built-in fuel tank, installed a new Mercury water separator, and installed new fuel lines & primer bulb from the engine to the tank. The internal engine fuel lines are new as of last Fall.
• Take it out for a test run and bamm, it runs great again.
• I feel comfortable now, so I took it out again this past Saturday and it ran fine for several hours until we stopped on a point and started it back up. I noticed the "flatness" as soon as I started the engine. Here we go again, same symptoms. I check the primer bulb and it isn't really "Hard", so I give it a couple pumps. As I start to plane off, it's still lacking power. A few seconds later it picks up and runs well again. This happens twice during the day.

One more thing I need to note is that the "beeper" beeps erratically sometimes when it's running flat (not always). It only does it at idle and it is not constant, nor is it a organized beep. If the engine is running correctly, there is no beep other than the normal start-up beep.

The "Flat" running almost always happens on start-up. So far it hasn't happened while running down the lake. It did happen once on Saturday when we were idling along. I felt it loose power and flatten out.

I'm stumped now. Could this be a fuel pump problem, or is it electrical? The fuel goes to the top carbs first, then it loops to the middle carbs, then to the bottom carbs. Could it be loosing fuel pressure and running out in the top carb first?

Any help?

 

[j_martin]

Re: 1988 XR4 - Faulty Fuel pump symptoms?

It could be a combination of things. The beeping is definitely not a fuel problem.

Take notes:

The thumb sized fuel filter on that thing will plug up in a heartbeat.

Water in the gas is bad bad bad bad bad bad bad bad

The idle circuit passages in those carbs are uber fine, and plug easily. The idle jet is actually metering the air.

Park the boat on a side hill so the end of the tank you can get at either through taking off something or through the filler is down hill. Then siphon from the bottom till it's clear of water and debris.

Either replace the filter on the engine, or better yet install a 60 GPH fuel/water seperator (RACOR) in the suction line.

The beeping could be ignition failure as the oil alert module gets one of its signals from the coil drive (usually #2) Intermittent failures are hard to find, but i'd first disconnect the kill wires and measure from either wire to ground with a good ohmmeter. It should be infinity unless the ignition switch is off or the engine is tilted up. If anything else, troubleshoot the short/leakage.

Grounds are critical. It's worthwhile to detail the ground wires on the switchboxes and coils.

Bad insulation on either the stator or the trigger will often manifest as cold engine problems, and go away when heat drives out any moisture. If there's any visible problem with either of those items, replace the part.

Further troubleshooting requires a DVA, spare parts, or deep pockets.

My engine's an 88 XR4, and I've played with it a lot. I can help.

hope it helps
John

 

[gaventure]

Re: 1988 XR4 - Faulty Fuel pump symptoms?

Thanks John.

I've already pulled the tank, drained it, cleaned it, and dried it out earlier last week. I also mounted a water separator (Mercury item with the separator that looks like an oil filter) and I replaced the small metal fuel filter with one that is see-thru glass. It shouldn't have any water or trash in the fuel now, but I'm still having the problem occasionally. I never know when it will happen. It has only happened at idle, and after it has been shut off, then restarted. It has never happened as I am going down the lake at speed.

If what you say is correct about the beeper, then it almost has to be electrical. The only time it beeps is when you first turn the key as it is supposed to do (before turning the engine over), and it does it erratically sometimes when it's idling, but only when it's running "flat". If it's running normal, it never beeps at all except when you first turn the key.

Sounds Electrical doesn't it?

I'll be out of town for the next 3 days, so I'll check back here when I return.

 

[j_martin]

Re: 1988 XR4 - Faulty Fuel pump symptoms?

It's probably misfiring on the cylinder that is sampled for the oil alert module, usually #2.

Sounds like a problem common to all, which could be the kill circuit, or the stator. Kill circuit is easiest to check when it's mis-behaving.

 

[gaventure]

Re: 1988 XR4 - Faulty Fuel pump symptoms?

Thanks again for your insight. I will return Thurs night and will start with troubleshooting the kill switch Friday. That would be a nice, cheap, easy fix. Of course the stator won't be if that's the problem.

If it is the kill switch, can you explain the problem? How does that cause the misfire? I ask because it only looses partial power, I would have assumed that a problem with the kill circuit would cause it to just die, completely cut power to all cylinders and loose all power at once, which would be much more noticeable.

I'm not doubting you at all, I'm just burning all of this into my mind so I can better understand how the electrical system works. I need a genuine manual, but haven't been able to find one for this model. Not even on Ebay.

 

[j_martin]

Re: 1988 XR4 - Faulty Fuel pump symptoms?

Thanks again for your insight. I will return Thurs night and will start with troubleshooting the kill switch Friday. That would be a nice, cheap, easy fix. Of course the stator won't be if that's the problem.

If it is the kill switch, can you explain the problem? How does that cause the misfire? I ask because it only looses partial power, I would have assumed that a problem with the kill circuit would cause it to just die, completely cut power to all cylinders and loose all power at once, which would be much more noticeable.

I'm not doubting you at all, I'm just burning all of this into my mind so I can better understand how the electrical system works. I need a genuine manual, but haven't been able to find one for this model. Not even on Ebay.

The kill terminals on the switchbox have some ignition type voltage on the, enough to bite you pretty good. The ignition switch grounds that, disabling the ignition. Assuming it goes right into the firing circuitry, you could also assume that a partial ground at that point could have widely varying but disabling type effects. Experience bears that out. A frayed wire on the tilt switch, for instance, is often the culprit in ignition failure.

I use service manual 90-97658-3 which covers v-150 - v225. It seems to be sufficient. Might be easier to find than the XR4-Magnum book.

Another thing to try is to disconnect the advance module. It'll take the tip off'n WOT, but you might not even notice. Most take that off and reset their WOT timing from 19* to 23* anyway, because when they fail shorted to ground, they usually grenade yer power head because that will advance the timing 20 degrees or so. The advance module is the larger black box on top of the engine with a ground and 2 wires going to the switchboxes.

I've done that, and I'm propped for 6000 rpm. Goes like a scalded dog.

Another thing to check is for interference with the trigger or trigger harness. It must be free to move through the entire range of motion, or it'll balk.

hope it helps
john