7.4L Bravo intermittent misfiring, & sometimes starts and runs fine?!?!?

[syuhas85]

Hello, we have a 89 Carver Montego with a 7.4L Bravo (MCM 454)

It just started having this problem where if you start it up it will act like its miss firing or running dead on a few cylinders,sometimes you can warm it up like that then shut it off and start it back up and it will run completely normal!?!

Yesterday we took the boat out and it ran ok no miss firing no sluggishness then when we came back to dock after being at the beach all day, i ran wide open at about 12mph she had absolutely no power....

What i have done so far:

New Plugs
New Coil
***Wires are about a year old and look brand new***
New water separator
New Fuel Filter for the Quadrajet Carb

I also took the Quadrajet off and took it to a friend of mine he put it in parts cleaner then cleaned it said everything looked fine I had one gummed up jet he cleaned it and put it back together.....

Now he didn't adjust my Carburetor because my boat is about hour away from house could it Need adjusted? could it be fuel is there a way I can take some fuel and get it sampled?

I know the motor runs perfect it just started this last week and I have changed just about everything besides:
putting a new distributor in and changing the ignition control module and doing the timing......

Any other ideas would be SO helpful cause I'm completely lost now.....


***OH one last thing we thought my brother mixed the plug wires up when putting new spark plugs in, so yesterday we flipped wire 1 and 3 around which they where right the first time but anyhow its like the misfiring went away but it didn't want to run so you know it was on the wrong cylinders so we stripped all the wires re did them matching the standard rotation on the distributor cap to the numbers on the engine and back to misfiring like crazy*** but it was weird to put the wire on the wrong cylinder and the the miss stops.......:facepalm:***

 

[syuhas85]

No Title

pictures of the motor

 

[alldodge]

but it was weird to put the wire on the wrong cylinder and the the miss stops

I would start over, start with number one plug and trace every wire to the plug, don't just say "yes that one goes to plug XX", trace them.
1-8-4-3-6-5-7-2
1-3-5-7 is on the left side, 2-4-6-8 on the right

If miss has not gone away, pull the plugs one at a time and see what they look like

Check the grounds on the motor and disconnect the tachometer wire (gray) from the negative side of the coil. Check all other connects to the distributer and coil.

Remove the distributor cap and check contacts, and pick up

 

[syuhas85]

I would start over, start with number one plug and trace every wire to the plug, don't just say "yes that one goes to plug XX", trace them.
1-8-4-3-6-5-7-2
1-3-5-7 is on the left side, 2-4-6-8 on the right

If miss has not gone away, pull the plugs one at a time and see what they look like

Check the grounds on the motor and disconnect the tachometer wire (gray) from the negative side of the coil. Check all other connects to the distributer and coil.

Remove the distributor cap and check contacts, and pick up

We did the wire part we stripped every where its stamped into intake manifold the Cylinder numbers and the firing order, then we just went off the cap gives you standard rotation and Left Hand rotation we put each wire one at a time doing standard rotation off the cap to the each cylinder number.

Ill run out today and see what the plugs look like and take a pic I will also take a pic of the inside of the distributor.

what will it do if i disconnect the tachometer from the coil?

 

[alldodge]

what will it do if i disconnect the tachometer from the coil?

The tach can go bad, and if it does, it will cause misfires, and can kill the motor. The tach is attached to the negative side of the coil and that is the side which is switched open/close by the ignition module. So it the tach starts shorting out it will kill the motor or cause it to miss

 

[Scott Danforth]

Bad ignition module also can cause similar symptoms.

However, back to the firing issue. You did not state you changed the cap an rotor. A corroded set of terminals in the cap will cause misfire.

I suggest a new cap and rotor, verify #1 on the dizzy, reinstall the wires, set the timing per the manual.

 

[syuhas85]

Bad ignition module also can cause similar symptoms.

However, back to the firing issue. You did not state you changed the cap an rotor. A corroded set of terminals in the cap will cause misfire.

I suggest a new cap and rotor, verify #1 on the dizzy, reinstall the wires, set the timing per the manual.

It won't let me fix it because I'm on my phone but I put new cap and rotor on but the sensor in the distributor kinda looked crusty I might need a new one but the new one don't look like mine they must of upgraded the part new part number 87-892150Q02.... Or might be the ignition module to is there any way to test it? I'm headed to the boat in a bit to try take the negative off the coil like alldodge stated, I do know there is two positives and two negatives on the coil I'll just take both negatives off coil see if it runs better. I'll update with pics when I get to the boat!

He attachments below the diagram is what my sensor looks like #8 but the new one looks totally different?

 

[alldodge]

The new one is of new design but still the same part.

As for the wires on the negative side, you only want to take the gray wire off (post 3), don't take any other wire off.

 

[syuhas85]

No Title

The new one is of new design but still the same part.

As for the wires on the negative side, you only want to take the gray wire off (post 3), don't take any other wire off.

I have 2 gray or yellow and 2 Reds on the coil I'll need to just figure out which one is for the tach. I posted the pic of it below that's the old coil before I changed it out but same setup with the new coil on, I also ordered the new sensor be here in two days I posted a pic of the Old it looks terrible when I took the Cap off there was no gasket on so evidently moisture built up inside.

Ill give updates once the sensor shows up and I install it, also is there anyway to test the ignition module to see if its faulty?

 

[alldodge]

The easy way would be to disconnect the tach wire at the tach

 

[syuhas85]

Ok so I'm back and she is still hesitating/ misfiring whatever the issue maybe badly..... I changed out the ignition sensor in the distributor and pull the negative off the tachometer neither fixed the problem. I also pulled the plugs they look fine.

SO

I'm down to timing which I'll take a timing light tomo and check that which will be kinda hard considering it needs to be idling normally to see if it stays steady at 8degree..

If that's not it its either the carb, vacuum or or dip-tube in the fuel tank.

Local marine repair shop, told me to pull the dip tube out of the tank and make sure the screen isn't plugged up...I asked him about the thunderbolt module failing he said he doubts that's the problem he's been there 30 years and has only had to change one, but that doesn't mean it's not the verdict yet.

I have a video of what the engine is doing but it's to big to post so once I get home I will upload it to YouTube and post it here.

 

[alldodge]

Don't think it's fuel but the easier way to prove it's not fuel is use a gas can. Remove the fuel line going to the tank and put it in a gas can with fresh gas.

 

[syuhas85]

heres the link to the video, the first rev it acts perfectly normal everything after that well it just falls on its face....... ignore the clicking noise its just the damn flappers

 

[NHGuy]

Definitely check the timing, and hopefully you went all over the ignition system looking for grounds. Clean and resecure all the grounds.
Disconnect the battery and clean up the master plug contacts with fine emery cloth and brass brushes.

If that is good do the Thunderbolt ignition troubleshooter chart. This verifies your controller works. (or fails which is unusual)

If all that is good, go back to the fuel system. Do the separate fuel tank test. If it runs better rev it some to see if it clears after some running. In fact, if you can run the boat under load you MIGHT be able to clear water out of the carb, so go for a ride if it runs well off the side tank.

Check for water in the fuel system by emptying the separator into a container. If you find any the carburetor might need to come off again because water is heavier than fuel and it sits in the bowl causing trouble. And the fuel tank would need to be cleaned.

If you find no water the carb could need a FULL clean and rebuild. There are some really small passages that can become clogged or restricted. The debris can be loose in there and not cause the problem at all times. The oldest carb guy you can find will be the best one to do the carburetor! They have seen it all.

Rebuilding your factory carburetor is preferable to buying a rebuilt quadrajet because there are differences from one to another.

But if you decide to replace the carburetor get the larger of the Edelbrocks, #1410. It's very adjustable and there are recipes for setup that you can use to set it up before installing.

 

[syuhas85]

No Title

*Update*

So the timing was at "4 degrees" Stock setting is " 8 Degrees" I got that on Dead money pic is below (and yes the motor is running in that pic). and It does not bounce around, only very little when engine starts acting up you'll see it advance to pick up the slack but stays steady at 8 degrees.

That still didn't fix my problem.

Before I went to fuel I figured I just start Yanking Plug Wire after Plug wire to see what spark Plugs sound like there the victims of this non-sense. I pulled every plug wire individually Only 1 cylinder made a difference when I pulled it......... "Cylinder #1" when I pulled this wire the engine started to struggle and shut down.... All the others you couldn't even tell if I pulled the wire off or not. "Basically Cylinder 1 is keeping her alive"


I went to the fuel line pulled the Red fuel line from the water separator to fuel tank off.... I blew that out with Air.

I then worked for the next half hour trying to get the dip tube/Elbow off......I cant get it to move I feel like i'm gonna brake the weld at the tank. I tried hooking the fuel line back up to the tank side and blowing air thru the fuel line but I'm guessing the dip tube has a Check-Valve in it because it wont let any air thru.


I still need need to get actual fuel line pressure and see where i'm at, also I have not done a compression test yet. I feel like it just needs more fuel I need to get this dip tube out!!!



The timing made the boat idle better and more responsive but still has a hesitation like its miss-firing same as video above when I rev it!

 

[alldodge]

I tried hooking the fuel line back up to the tank side and blowing air thru the fuel line but I'm guessing the dip tube has a Check-Valve in it because it wont let any air thru.

Yes, the tank has an anti siphon valve on it and should stop air from going in.

 

[syuhas85]

Yes, the tank has an anti siphon valve on it and should stop air from going in.

Thanks for the Info I figured since it wouldn't let air go thru, I'm gonna try to at least get the anti-siphon valve off I can then blow air thru and get the debri back down the dip-tube. I cant get the whole elbow/Dip-tube out it wont budge and I don't want to break the tank there's no way to get that 125 gallon tank out if it breaks besides cutting the boat up its dead center of the boat under the aft cab.

I bought a new 3/8" Nylon gas line to go from water-seperator to gas tank just for the hell of it, it was cheap from Napa, and I'm gonna take a little 1 gallon tank of fuel with me see, Least this will tell me if its fuel related or not.


If she starts spitterin and spatterin with running off a different tank then I'm back to square one:facepalm:


I'll give updates when I get back home tonight.

 

[alldodge]

A one gallon can it's going to get the 7.4 very far, mine does good and it gets 2 to 2.5 miles to the gallon.

 

[syuhas85]

A one gallon can it's going to get the 7.4 very far, mine does good and it gets 2 to 2.5 miles to the gallon.

I hear ya, I'm not taking it out the hesitation is instant so if it goes away with runing it on that one gallon can then I know it's in the dip-tube/gas tank....

 

[syuhas85]

Ok so update well one I'm drunk two I'm headed to the Kelly's island on friends boat lol..... But I did take the check valve off it works fine I blew air down the tank u can hear the fuel bubbling.... I attached my personal gas tank for my lawnmower it didn't change a thing still hesitating sooooo now I'm down to mechanical fuel pump or carb what I did notice!!!!

Is when you rev the motor it's smooth till the back two barrels open then you can see the 4 barrels or back two barrels open and they flutter with be hesitation....there is no hesitation till you floor the boat and you see the back two barrels open in guessing the carb needs rebuilt