7.4 troubles

[marcdups]

Good day everyone. Am having an engine kick my butt !!! OK, here we go, 7.4 L mercruiser 1997, spark yes, fuel yes, did all mercruiser tests, even by passed boat's wiring harness and went straight to battery from coil. Engine will not fire !! now comes compression test, according to manual 150 psi
1: 130 2:140
3: 120 4:120
5: 127 6:115
7: 150 8:140
Now on # 7 cyl, found full of fuel, maybe stuck valve, redid compression test on # 7 after doing all cylinders ( while fuel kept coming out of # 7, until dry )and found 115 PSI. Am wondering if am not getting enough compression to have detonation, am stumped, help would be nicely appreciated, also could be something that I totally missed. Thanks in advance for any and all help I can get, marc

[gcarter]

Good day everyone. Am having an engine kick my butt !!! OK, here we go, 7.4 L mercruiser 1997, spark yes, fuel yes, did all mercruiser tests, even by passed boat's wiring harness and went straight to battery from coil. Engine will not fire !! now comes compression test, according to manual 150 psi
1: 130 2:140
3: 120 4:120
5: 127 6:115
7: 150 8:140
Now on # 7 cyl, found full of fuel, maybe stuck valve, redid compression test on # 7 after doing all cylinders ( while fuel kept coming out of # 7, until dry )and found 115 PSI. Am wondering if am not getting enough compression to have detonation, am stumped, help would be nicely appreciated, also could be something that I totally missed. Thanks in advance for any and all help I can get, marc
Isn't it true you're going to get low readings on a cold engine?

[marcdups]

George, agreed, compression readings are something for reference, what worries me is that in removing plugs for compression test, #4,6,7 were wet and after getting rid of fuel in # 7 cyl comp went from 150 to 115, last time I got engine to start, a bunch of black and dark gray smoke coming out of x haust and engine running rough, could not pinpoint problem b4 engine died. Even switched out ignition sensor in dist even though it tested ok, the last thing that remains on ignition system is Ign module but mercruiser tests show all ign components to be in working order. even changed plugs although they were new and tested fire to every plug prior to installing. I thought fuel was dead or water, drained filter and looks fine, even tried to start with engine crack( starting fluid), carb has been remanufactured, new fuel pump ( mechinical ), new plugs, cap, rotor, fuel system, only thing left to my knowledge is compression or timing, but at 10 BTDC rotor is in position and piston is nearly at TDC. what gives !!!! :bonk::bonk: that is probably what I need, lol

[gcarter]

Do you mean no cylinders will fire?
One moment I'd like to forget was when I wired the plugs to reverse rotation because I was reading wrong direction on top of Merc distributor cap. Of course engine would barely run.

[marcdups]

George, after getting the carb re manufactured, engine cranked up like a champ, and ran with no issues, at idle and not under load, even revving engine at dock was ok. Here is prob what I omitted, at times it seems like the engine is not aspirating fuel and I have fuel in left in the carb intake venturi and a light whoooo sound coming out of carb like some intake valves are stuck open maybe, which I would think would show up in even a cold compression test, but then again might be flooding the cylinders, specifically # 4,6 & 7. Not sure, am at a loss as I have gone thru the manual and all specific tests appear to be within specs except compression, hot or cold there should not be more than 10% difference between them, right ??
Baffled here on a beautiful sat afternoon, :nilly:

[marcdups]

And I bet the solution is an easy one, wondering if the ignition module breaks down when in demand, but tend to have spark at all plugs when testing with an in line spark tester, no, poodle ( just in case you would ask that ) the spark tester is not from wal mart, but Mac tools.

[mrfixxall]

if you have fuel left over in cyl #7 after the exhaust stroke it will force it back into the intake and fill other cylinders with fuel..pull all the plugs,unplug ignition module so theirs no spark and turn the engine over with the carberator in full throttle mode with the secondarys open...leave it sit like this for a few hours so the cylinders can dry out,,sounds like you flooded the $hit out of it and with the fireing order wired backwards you probably have fuel in the oil also..pull the dip stick and smell it,if it smells like gas the you need to change it rite away because it wont only thin out the oil and distroy the engines bearings but it could explode internally and cause a big fire..

Go over the fireing order again and throw new plugs in it and leave the throttle in wot with the choke proped open. when the engine starts pull the throttle back and let it clear out and it should be ok..

[marcdups]

Gonna run over to boat and take mrfixall"s advice, may even put new wires for ****s and giggles and will update in the morning, thanx guys

[DonziJon]

I'm a simple person so I have a simple possability. Maybe the distributer is in backwards. TIMING. Just a possability. FIX might be on to something as well John

[marcdups]

OK, call me crazy but here goes a theory and will tell you why !! Remove plugs per mrfixall's advice, no fuel in cylinders and new plugs dry, re install plugs, start engine at WOT and back off from there, engine running rough at @ 1500 rpms, drop cylinder test reveal all cyls firing, fuel mixture extremely rich and engine has a hard time scavenging air. Now here comes the crazy thought, too much back pressure as it seems like this whooo sound coming from carb must be back pressure coming through intake valves, now remove all plugs on port side and roll engine over, without plugs on that side, the back pressure through carb goes away. Next I removed the riser looking for blockage, although there is some corrosion, passages are somewhat clear, re install and crank engine, with engine at high idle the temperature of the riser was 97 deg where as the down tube to Y pipe was 123 deg. on starboard side riser temp 97 deg and down tube 93 deg, tomorrow removing the down tube and attempt to find blockage in Y pipe, either from older flappers or something !!
Now any one think I lost my mind ???

[mrfixxall]

if the engine isnt up to par on the running condition the card will make all kinds of funny noises..sounds like you may have carb issues or just mabe your timing chain may have jumped a tooth..line up your crank to zero on the harmonic ballancer and ck the dist rotor position..should be a tad off of cyl # 1 or 6..

[Donzigo]

Check out the Hobbs low oil pressure switch on the rear port side of the engine. I had a similar problem with my 1997 7.4 and after pulling my hair out, it was the switch.

My guess is that you are running a Holly carb. Look of the front of the carb and make sure the 1/4 pipe is covered with the rubber cap. I had my carb rebuit once and it also drove me crazy and my pal comes over and we go for a ride in the bay and I was getting frustrated and he just smiled. Then, he put his finger on the pipe and she ran great. We still laugh about that one.

[MOP]

Don't worry about the 115 cylinder just yet, I have seen very low compression on a fuel washed cylinder that tested decent after a little run time. The sound you describe sounds like it may be a bit off time, pull No 1 plug bump the engine over slowly with your finger in the hole until it pops your finger out. Look to see where the timing pointer is in relation to the timing marks, the distributor may have loosened and moved a tad.

Phil