1991 Merc XR4 issue

This topic has 4 replies, 3 voices, and was last updated 8 years, 5 months ago by big.ed.

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December 16, 2017 at 8:59 am #346328

big.ed
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Hey guys and gals. I am hoping someone can help me. My cousin has a 1991 Mercury XR4 150hp when running at or above 5k rpm, the motor will suddenly drop to 4k rpm and start missing. Then, just as suddenly, it will go back to 5k and run fine. We have had it in the shop, and the dealer couldnt find anything mechanically wrong. They even reached out to the local mercury rep and they were no help. We rebuilt the fuel pump, and have purchased new coil packs, but have yet to put them on. The compression is great in each cylinder, and the fuel is non-ethanol and new fuel lines were installed 2 years a go. We are not sure what the deal is, but we think it is electrical. It is a carborated motor, so no EFI. The only thing we can think of is either a coil packs bad, or a voltage regulator. It has new plugs as well. Thank you in advance for any help or suggestions.Sent from my SM-G955U using Tapatalk

December 16, 2017 at 3:50 pm #346348

chvy91bowtie
Participant

I had an old force that did the same thing chased it for 2 years. Came across some coils on eBay and figured why not. Installed them and never had an issue again. Sent from my iPhone using Tapatalk

December 17, 2017 at 3:15 am #346365

big.ed
Participant

chvy91bowtie wrote:I had an old force that did the same thing chased it for 2 years. Came across some coils on eBay and figured why not. Installed them and never had an issue again. Sent from my iPhone using TapatalkThanks, hopefully that will help. You would think the dealer would have checked all the electrical too since they had the boat for 2 months.Sent from my SM-G955U using Tapatalk

December 17, 2017 at 5:43 am #346383

oldtimer57
Participant

coils can cause that. I had a 92 XR6 that had a coil that developed an intermittent problem. NOT easy to isolate. You can replace all 6, which is not real cheap. You can try just one new one and moving it from one cylinder to the next to see if the problem goes away. Id be more likely to take an inductive timing light, and clamp it around the plug wire going to cylinder #1, then test driving. Watch the strobe end carefully when it starts to miss. If it continues to pulse regularly, stop, move to cylinder and continue. When you find the plug wire that shows an intermittent spark based on intermittent strobe flashing, replace that coil (or plug of course) and sometimes you might have a bad wire, switch box, etc…

December 17, 2017 at 6:08 am #346400

big.ed
Participant

oldtimer57 wrote:coils can cause that. I had a 92 XR6 that had a coil that developed an intermittent problem. NOT easy to isolate. You can replace all 6, which is not real cheap. You can try just one new one and moving it from one cylinder to the next to see if the problem goes away. Id be more likely to take an inductive timing light, and clamp it around the plug wire going to cylinder #1, then test driving. Watch the strobe end carefully when it starts to miss. If it continues to pulse regularly, stop, move to cylinder and continue. When you find the plug wire that shows an intermittent spark based on intermittent strobe flashing, replace that coil (or plug of course) and sometimes you might have a bad wire, switch box, etc…Thanks. He bought all new coil packs. We just ran out of warm weather to install and test. We will install them and run it. Hopefully that will fix it.Sent from my SM-G955U using Tapatalk
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