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SUBJECT: # 25069: Mercruiser help, backfiring

Submitted by Ed (205.188.208.102) from SOUTH CAROLINA on 3/9/2003 7:09:00 PM

Can someone please give me an indication of where to go next? The motor is a 140 HP mercruiser (4 Cylinder GM). It is a 3 year old repower motor. It misses and backfires, but only after it has been run for 30 to 60 minutes. It will then run along fine for several minutes, then act up again. So far I have replaced the water separating fuel filter, had the timing checked, replaced the distributor and rotor, replaced all the spark plugs, checked all the wires. There is no water visible in the oil. What do you think? What should I have done next? It seems like water in there somewhere. Could it be carburation? A faulty head gasket? Water in the gas despite the water separating gas filter? Thanks so much for your input about what I should do next.

  1. 3/9/2003 9:33:00 PM Submitted by Greg (24.188.112.92) from NJ says engine
    QUESTION: ----Does this engine have a point & condenser ignition system? If so, I would suspect either the condenser or the ignition coil, (after they get hot).---Just my opinion. ----Greg

  2. 3/10/2003 8:00:00 AM Submitted by Tom (24.131.101.68) from OH says Engine backfiring and missing
    Sounds like what I had. I finally figured out that the coil would break down after it got hot. I agree with Greg's opinion.

  3. 3/10/2003 8:21:00 AM Submitted by Ed (128.23.45.154) from SC says engine backfiring
    Greg and Tom, Thanks. That gives me somewhere to head with this problem. Should I replace the coil or the condenser first? or do both at the same time? Thanks, Ed

  4. 3/10/2003 8:36:00 AM Submitted by Greg (24.188.112.92) from NJ says Engine
    I would purchase a new set of ignition points and a condenser together with a new ignition coil. Purchase only HIGH QUALITY MARINE PRODUCTS. Don't be cheap!!!! Install the points and gap them according to the specifications in the engine manual. After you install the all the items, run the engine at idle at the dock for 30 minutes to see what happens. Check the base timing with a timing light. Get the engine compartment up to operating temperature. While you are at the dock, listen for any vacuum leaks, (hoses on the intake manifold, carb base --etc) A vacuum leak can cause a very lean mixture and this can also cause a backfire.Check the condition of the PCV valve. ------Just my opinion. --Greg

  5. 3/10/2003 3:27:00 PM Submitted by Ed (128.23.7.239) from SC says coil
    Greg,

    Thanks, you have pointed me in good directions. The cap, rotor and points are already new and the timing is dead on. I have ordered a new coil, it should be here in a few days. Then some time at the dock and in sea trials. One other thing--the backfire only happens when there is a load on the engine, never at idle. Does that point to anything else? Thanks again.

    Ed

  6. 3/10/2003 7:51:00 PM Submitted by Greg (24.188.112.92) from NJ says Backfire
    YES it does point to a vacuum advance or a mechanical advance in the distributor. Put a timing light on the engine. Disconnect the vacuum line to the advance and plug the hose. Point the timing light at the timing marks, and raise the idle. See how much the marks move in the light. Now connect the vacuum line, and see if the marks move further. You could have "late timing" under load. ----Greg

  7. 3/10/2003 7:56:00 PM Submitted by Greg (24.188.112.92) from NJ says Question
    Does this engine have a ceramic ballast resistor in the ignition circuit? Sometimes it is mounted on the outside of the ignition coil, sometimes it is part of the coil, and on a car, it could be found on the fire wall. The fact that the engine needs to run for a lenth of time, makes me go back to electrical issues. ----Greg

  8. 3/11/2003 4:15:00 PM Submitted by Ed (128.23.47.70) from SC says backfiring
    Greg, I can't tell you how much I appreciate your input. This problem seemed to have my mechanic stumped. He kept saying 'water in the gas" but I didn't see how that could be with fresh gas and the water separating fuel filter. Also no water in the bowl of the fuel pump when I checked. I originally thought about electrical, but was being led astray. His ideas have not been helpful. I will let you know what I find out after I replace the coil and do what you have suggested. Ed

  9. 3/11/2003 8:28:00 PM Submitted by RickK (205.188.209.109) from VA says Last resort.........
    All of the above are right on target, but replacing parts is expensive. It might be worth your while to have it professionally diagnosed.

    Last resort could be the harmonic balancer. I had an old car that I replaced absolutely everything on. It turned out to be the harmonic balancer. The slightest acceleration would cause it to backfire. Very frustrating and expensive paying for all the parts I didn't need.

    RickK

  10. 3/12/2003 5:17:00 AM Submitted by Greg (24.188.112.92) from NJ says Engine
    Ed, you have stated that the engine needs to run form 30 to 60 minutes for the problem to surface. Lets take a few minutes to explore the water issue. If you had a "little" water in the fuel, after 30 to 60 minutes when the engine is hot, the water would turn to steam when it hits the hot combustion chamber. If you had a "large quantity of water" that would be a totally different issue. QUESTIONS: ----What kind of fuel pump is on this engine? ----Mechanical or Electrical? ----Are they providing good pressure and capacity? Yes, the pump must be tested for "capacity". ----Are you starving for fuel at high speeds or under load? ----Is the fuel pick up from the tank clean? ---I think there is a valve in the line on the tank end.----- EXAMPLE: ---My wife and I own a 1999 27' Doral Cruiser with a 7.4 MerCruiser V8 & a Bravo 3 drive. Last year on the Hudson River while cruising at 3400 rpms, I noticed that the tach. was "searching" (moving slightly), yet the speed of the boat remained the same. This is a classic symptom of an electric fuel pump going bad. I had the pump replaced the next day, and the problem was solved. The engine also had more "pep" with the new pump. Put the water issue to rest, by taking a sample of the fuel form the tank. Put it in a clean glass jar, and let it sit for a period of time on a flat surface. If water is in the fuel you will see a line between the two substances. Look at the exhaust when the engine is "acting up". ---Do you see a lot of steam? ---If so, you might have a head gasket problem, but if this was the case, you should also have a problem when the engine is cold. QUESTION: ---Did you ever overheat this engine? Does the valve train move easily? What does the engine oil look like? ---I hope this information helps. ------Greg

  11. 3/12/2003 8:30:00 AM Submitted by Ed (128.23.208.17) from SC says fuel
    Greg, The fuel pump is mechanical. There is no backfiring for the first 30 to 60 minutes even when the RPMs are up at load. The engine was never overheated. The problem will come and go after the first 30 to 60 minutes, but then seems to get progressively worse. The oil is pristine. Unfortunately I know what water in the oil looks like, having had a head completely corrode through 3 years ago. Chocolate milk is not supposed to be in one's crankcase! I have not looked at the fuel pickup in the tank.

    Ed

  12. 3/12/2003 8:49:00 AM Submitted by Greg (24.188.112.92) from NJ says Problem
    I still think that this is an electrical problem, but we need to keep the options open, because strange things happen on boats and cars. Is it possible that you have a problem with the ignition switch? Do you have a lot of keys on the key ring, and in rough water the switch is making and breaking contact? With the engine running, shake the key and see what happens. Look at the back side of the switch to see if all wires are tight. Could you be losing the electrical ground? Check the ground wires on the engine. Make sure all connections are clean and bright. Make sure the battery connections are clean and tight. If you have a "side terminal battery", make sure the cables are tight. Some battery cable bolts are TOO LONG, and they bottom in the battery, before the cable becomes tight. You might have to add a steel washer to the bolt. ------Greg

  13. 3/12/2003 9:11:00 AM Submitted by Greg (24.188.112.92) from NJ says Additional Information
    There is the possibility that you develop a vacuum leak around the base of the carb or the intake manifold after the engine gets hot. When the engine is hot spray some water around the gasket areas to see if the liquid gets sucked into the space. This would upset the air / fuel mixture. What is the condition of the spark plugs? Are they clean or dirty? Are they overheated? Are they the correct plugs for the engine. Are they the correct heat range? When the engine breaks up, do all the cylinders break up, or is it just one cylinder? Does this engine call for resistor plugs? Make sure you do not have carbon tracking on the inside of the distributor cap. If your engine has "blow by", and the PCV valve cannot remove all the "blow by", it is possible for crankcase fumes to work there way up the distributor shaft and cause carbon tracking in the cap. Make sure you have the correct PCV valve and the PCV system is operating properly. It is obvious that something is occuring that is heat connected. A cracked distributor cap could cause something like this to occur. When you find the cause it will be "simple". Keep us informed. I am very interested in this problem. -------Greg

  14. 3/12/2003 9:27:00 AM Submitted by Ed (128.23.208.17) from SC says things
    Greg,

    All good comments. The plugs are new, correct and replaced after the problem started. The distributor cap and rotor and new (so inexpensive I figured what the heck) The ignition switch I hadn't thought about. I will check, but there are not a lot of keys on it. I will check the connections. I will double check the battery cable terminals and the grounds. Thanks for the tip about spraying some water aroound the base of the carb. I had thought about a leak there, but didn't know how to check for it. Unfortunately all this is going to have to be this weekend, because I work late into the evenings. Now I have some things to try out. Please keep the ideas coming.

    Ed

  15. 3/12/2003 9:46:00 AM Submitted by Greg (24.188.112.92) from NJ says More ideas
    Ed, I want you to think back to when this problem first showed up. Was anything installed in the boat just before this problem occured? ----For example, a "Starter" on the engine! On a land vehicle, the ignition switch is powered by a wire from the starter solenoid. Make sure the connections are clean and "tight". Examine the wire from the engine compartment to the ignition switch. Look for loose wires in the engine compartment. A loose ground in the electrical system could cause this problem. Take your time with this troubleshooting. Could this be a "stuck" intake valve? If everything else checks out "ok", and the problem still exists, with safety goggles ON your face, dribble,(very slowly), some Marvel Mystery Oil into the carb with the engine at a high rpm. DO NOT STALL the engine. If it is a valve stuck in the guide, the MM oil should free it. ------Greg

  16. 3/12/2003 9:50:00 AM Submitted by Greg (24.188.112.92) from NJ says Wires
    Ed, on your first post, you said that you checked your wires. I am assuming you are refering to the spark plug wires. Carbon wires will break up when they get hot. This could be a simple fix. ------Greg

  17. 3/12/2003 12:48:00 PM Submitted by Ed (128.23.208.17) from SC says parts
    No new parts before it started. Just one day cruising along and the backfiring started. You have given me so many leads that I have to get the cowl off and start one at a time. I am anxious to get to it. Starting with the ignition switch, then the battery terminals, then all the other connections, then to the carb. I foresee a lot of fun figuring it out. Like you said it may be something quite simple, then I can slap myself in the head and say "Ed, you idiot".

  18. 3/12/2003 5:08:00 PM Submitted by Local Motion from NY says my take...
    ..not battery terminals or anything like that. Motor will run fine without a battery even connected.

    May be a bad wire for ignition, ground or +12hot, usually purple.

    Follow your ignition wire, usually yellow with a stripe right through your main engine harness connector, the BIG plug. These connectors/plugs are big-time notorious for giving you an intermittant ignition.

    Not the coil getting hot or it wouldn't clear up right away. What it may be that would clear up after giving it a short break is motor overheating due to the water intake hose collapsing at high speed and uncollapsing when you slow back down and cooling the motor back down. This problem is often over-looked and often left unchanged because it can be hard to get to. Check that hose to make sure it is not soft and easily collapsed under suction.

    Hope this helps, Greg.

  19. 3/12/2003 7:42:00 PM Submitted by Greg (24.188.112.92) from NJ says Some other things
    If you still have this problem after you check all the above,--- look for the following: ----1.) Broken valve spring, 2.) Make sure the choke on the carb is coming all the way off, and staying off, 3.) Make sure all the intake bolts are tight, 4.) Is it possible that you might have a hair line cylinder head crack that opens up after running for awhile, dripping a small amount of water into a cylinder, causing a backfire? If this is the case, look for a spark plug that is VERY clean, because it is being steamed. ---------Lost of luck. ------Greg

  20. 3/15/2003 1:46:00 PM Submitted by Ed (152.163.188.38) from SC says What I learned and did Saturday morning
    Did: took some gas from the tank and put it in a clear glass to stand for several hours

    Learned: No water/gas separation

    Did: took the ball valve connector off the top of the fuel tank, blew some air into the tank (lung power) until I heard it bubbling in there

    Learned: There was a piece of something right at the ball valve that I took out

    Did: looked at all the electrical connections I could find that had bolts or screws and cleaned and tightened them as necessary (or even if not necessary)

    Did: Started it up (started just fine and idles and takes some low throttle like a champ. fooled around with connections I could see while it was running. The ignition switch seemed fine, when i pulled the connection the motor just shut off.

    Learned: When I jiggled the connection onto the coil (not the wire to the distributor) there was some missing All I had to do was move it just a little bit and there was a problem

    Did: Took off the connections, cleaned and applied dielectic grease and reattached

    Did: pushed on the switch that is the reverse cutoff, and it seemed to do what it is supposed to. I cleaned that area up

    Did: Looked at that big connector that Local Motion is talking about (before I read his post) I will have to get back to that. I did push on it a little, but it did seem tight

    Did: Changed the oil and filter Quicksilver 25-40

    Learned: It was starting to rain as I put the last quarts in and it was time to cover things up and clean up the mess

    Did: Cleaned out my toolbox. Yucky stuff in there

    Did: Took a shower

    Did: Read your posts and wrote this

    Learned (Again): The folks on this board are very willing to help. Thanks guys.

    Ed

  21. 3/15/2003 8:35:00 PM Submitted by Greg (24.188.112.92) from NJ says Boat
    Ed, I hope that everything is "ok" with you boat. If you need more help, you know where we "hang out"! ----Have a great day! -----Greg

  22. 5/22/2006 10:32:00 AM Submitted by Need help 911 (70.128.184.227) from TX says Sunken Boat
    My boat sank when I was out last week. It was making a horrible sucking sound. I lifted the cover and saw that the engine was competely covered with water and it was spitting out foamy, caulk-like stuff. I assume this was oil saturated with water. This is when I realized that the plug was missing. I managed to find someone to tow me into the dock before completely going under, I brought the boat home and filed an insurance claim. The mechanic is stating that there is a crack in the engine. I took excellent care of my boat, always winterized it. I have never had a problem before. Is it possible for the engine to crack for any other reason than freezing? Where can I find documentation of other possibilities? The crack is very small, the mechanic says it looked like there was debris, but that it is clear there was not water in the engine.

  23. 5/29/2006 7:06:00 PM Submitted by dnova (216.45.200.164) from NY says mercruiser backfire
    do you have a high energy ignition? if so you need to check youre alternator output it should at about 14.2 volts. if not it may cause the igniton module to missfire,the reason you dont realize this is becuase there is enouogh voltage to keep the batteries charged.let me know if it works out

  24. 6/4/2006 9:00:00 AM Submitted by Chad (70.238.107.75) from MI says Ford 2..3 litre / 4 cylinder
    I have a 1987 glassport runabout (16.5 ft) and last summer I at the end I had this problem with what seemed to be somewhat of backfiring, sometimes it would flat out stall.....I had a friend on the boat with me that is very handy around engines and he suggested that I change the Plug wires because when he touched one he was "shocked" I am in the process of switching wires , but I just wanted to see if this sounded like a common problem. It does run fine at slow speeds or in neutral, its just the higher speeds is when it starts to "talk back" Please help if you can, this is the 1st boat that I have owned.

  25. 6/22/2006 5:20:00 PM Submitted by Benjamen (70.129.226.34) from KS says 1984 Mercruiser 3.8 litre
    My friends boat just up and quit on him a couple of days ago while cruising at high speed. He was able to use the trolling motor to get it to shore and then hand tow it back to the launch. When he called me to help him fix it, it was having some of the same troubles Ed mentioned with his. Wouldn't start, only backfire. We checked coil output, seemed hot. Checked a plug to see how much fire it was getting, seemed good. After reading some of the posts here I thought it might be the coil, so we replaced that. We were in the process of replacing the points when we realized the rotor was split right up the side. This allowed the rotor to spin just slightly on the shaft and totally screw up the ignition timing. Timing was our first suspicion, but because the distriubutor was tight we didn't figure that was it. Turns out it was, just for a different reason. Replaced the rotor and it runs like a champ.

  26. 1/16/2007 1:41:00 PM Submitted by Doug Cowan (192.76.82.89) from FL says Mercruiser 7.4L Inboard Won't Turnover
    New batteries. Port engine cranks fine. But I only get a clicking noise on starboard engine. Is this possibly a solenoid issue?

  27. 11/29/2007 11:44:00 PM Submitted by Barry Tanner (149.135.107.154) from Australia says Mercruiser pulsating noise from return line from fuel pump to carby
    I have a pulsating noise coming from the fuel pump return line to the carby on one motor ,I have two 4 cyl. 165 fitted to a Bertram 25. This is the plastic line that apparently returns excess fuel/vapours to carby.

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