'91 Mercury XR4 compression and spark issue? Please help.

[seo515]

Hi everyone. I have been reading this site for the past 2 years and have gotten a lot of great help with my problems. Now, I finally have a problem that I can't seem to find answers to and would like your collective help on my issues.

I recently purchased a bass boat with a '91 Mercury XR4 150. Unfortunately,the seller was nowhere near a lake and so I was not able to test drive it(I know, not a good idea) or have it cranked up on muffs during my purchase. Buyer beware, right? Well, I bring it home and cranked her up. She started right up, but immediately starts rapidly beeping non-stop(not overheat buzz) unless I turn off the ignition. Based on what I've found on this site, it seems to be related to the notorious failing oil injection system. I took the boat to a local marine shop to have the engine inspected. They charged me $90 to simply do a compression and spark test? Anyhow, I was told that my compression readings were 130, 70, 125, 130, 130, 125. I bought a compression gauge at AutoZone just to verify for myself. #2 cylinder was, indeed, low. The mechanic told me I would need a rebuild. Unfortunately, I don't have the money for a rebuild.

Reading the forums on related issues, everyone seems to recommend a de-carb. So, I put Seafoam in my fuel and sprayed Deep Creep into my carbs and sprayed some deep creep into the cylinder. I mixed the fuel tank with 50:1 oil mix since the oil injection system is having problems. After a couple of days, I tried another compression test. But this time, the motor cranked with the #2 spark plug out. Is this normal? The engine fired right up while full throttle. I immediately turned the engine off and read the compression gauge. It read 130psi. My question is...is the motor supposed to crank with one spark plug out? And is my compression reading of 130psi a false reading because the engine cranked up rather than turning it over several times? Sorry for the lengthy post. I just wanted to try and clearly explain what is going on with my issue. Any help would be greatly appreciated.

 

[Maxz695]

Re: '91 Mercury XR4 compression and spark issue? Please help.

A running engine I don,t believe will give you a proper reading. Disconnect the wires after marking where they go and the insert the compression gage turn the engine over several times to get you proper reading.

 

[jaredlucky16]

Re: '91 Mercury XR4 compression and spark issue? Please help.

or just hit the kill switch

 

[seo515]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Is something wrong with wiring if the engine starts with a plug out? I can't turn it over a few cranks for a reading because the engine starts right up. As jaredlucky16 suggested, I guess I could use the kill switch to disable it from starting. But now I'm wondering if there is now an electrical issue allowing the engine to start while the compression gauge is connected to the #2 cylinder.

 

[Maxz695]

Re: '91 Mercury XR4 compression and spark issue? Please help.

an engine can run on one of 2 cylinder or 2 cylinders of 3 cylinder even with a plug removed. Disconect the Plug wires from all making sure you know where they go back to

 

[seo515]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Maxz695, thank you. I'll try removing all the plug wires to ensure it won't fire up. Do you have any idea about the constant rapid beeping? I read other posts stating that the beeping intermittently turns every few minutes or when the conditions arise. But mine beeps non-stop from the moment the engine cranks. Is this(rather than the typical overheat "buzz") an indicator that my engine is overheating?

 

[j_martin]

Re: '91 Mercury XR4 compression and spark issue? Please help.

The rapid beeping is coming from the oil alert module. it means
1. The oil is going down in the engine mounted tank
2. or the tank sensor is bad
3. or the oil pump is not turning
4. or the oil pump rotation sensor is bad
5. or the spark signal from #2 is bad
6. or the oil alert module is bad.

You need to get the manual and go through the systematic diagnostics in it to discern which part is bad. Meanwhile, mix oil in the gas and ignore the beeping.

On the compression test. There should be a guarded toggle switch on the back of the controls. It is designed so the loop of a dead man lanyard will go between the guard and the switch such that if the driver leaves the boat with the lanyard, the switch will throw downward and kill the engine, and the boat will wait for said man to swim to it and climb aboard. That's the kill switch mentioned in previous posts.

If you disconnect the spark plugs, make provision to ground them, or overvoltage could wipe out some ignition parts. If you throw the kill switch downward, and try to start the engine, it should not fire. If it does, there's something wrong with the kill switch.

Compression is taken at cranking speed. At high speed you'd get compression with a wooden plug for a piston.

Oh, and don't hold your breath hoping SeaFoam is going to bring your compression back.

hope it helps
John

 

[Maxz695]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Excellent post J

 

[seo515]

Re: '91 Mercury XR4 compression and spark issue? Please help.

John, thank you for such an in-depth response. I have ordered a repair manual and expect it in a couple of days. I know that these forums tend to provide great information on troubleshooting from experienced boaters. The additional advice to ground the plugs to prevent electrical problems is a life saver. I wouldn't be able to afford additional expenses and failures due to my own mistakes. Then would it be OK to simply toggle the kill switch and put a compression gauge on the #2 cylinder for a reading?

About the SeaFoam, I guess I was just hoping it would help to get the compression up a bit based on the posts I've read. The engine was apparently not well taken care of and so carbon build up probably exists. I guess it was still a good idea to de-carb it since the engine is 20 years old? I guess my only option with the low compression on my #2 cylinder would have to be to have it rebuilt by a mechanic since I am not experienced in that type of work.

 

[j_martin]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Just be sure the kill switch works before you trust it. (As I described) When I do a compression check, I pull all the plugs, ground all the wires, and use a remote starter switch which will crank the engine without the ignition switch being on. All plugs out is to take a lot of load off the starter motor.

If it didn't have water to it when it accidentally fired up full throttle, you probably wrecked the impeller, and maybe the housing on the water pump. A couple of seconds dry at that speed sorta plates the impeller onto the pump sleeve.

 

[jrocksho]

Re: '91 Mercury XR4 compression and spark issue? Please help.

You did you sprayed deep creep into cylinder? If you're losing compression, more than likely stuck or bad rings, then any oil like deep creep sprayed in cylinder will indeed raise your compression until it burns off. My truck is suffering from this, running on 7cylinders now. One cylinder is about 30% lower than rest, if i squirt some oil thru spark plug hole, my compression reads normal until oil quits sealing, which isn't long. The oil is actually sealing the compression leak temporarily. Any engine can run with plugs out. Compression test does not require 12v, i usually flip my kill switch to kill spark while testing, but there are other ways to kill spark and keep 12v to starter.

 

[jrocksho]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Disconnecting all wires will work, but i wouldnt recommend that. Your wires will still be sparking, make sure your not enclosed with fumes. Instead disconnect power from coil packs or kill switch works also.

 

[j_martin]

Re: '91 Mercury XR4 compression and spark issue? Please help.

jrocksho, with all due respect
1. This ain't a truck we're talking about
2. Loose spark wires are a problem well discussed already in this thread
3. 12 V doesn't even get close to the ignition system on this engine. Kill technique also was discussed.

 

[mercman2]

Re: '91 Mercury XR4 compression and spark issue? Please help.

I agree as well j. This is usually when I am doing my spark test I just leave my spark checker hooked up while I complete the comp test, or the deadman switch works as previously mentioned. And yes they will run with a cylinder or two unhooked.

 

[seo515]

Re: '91 Mercury XR4 compression and spark issue? Please help.

Ok, so I followed the advice of disabling the kill switch to prevent the engine from starting while doing a compression check. Maybe it was due to doing the de-carb, but the compression did increase in my bad cylinder from 70lbs to 85lbs. I guess at this point, there is nothing else that can be done except for a rebuild. I'll post questions on that in another thread. Thank you all for your advice!