Mercruiser MAG MPI 5.0 No start

[aggiekid]

I rebuilt a mercruiser 5.0 MAG MPI. The get fuel in the fuel railing, and sparks at the spark plugs. I cannot however get the engine to start without manually controlling the throttle body. If I don't open the throttle manually,after several tries to start the engine I hear a loud bang from the throttle body; as if the engine needed air. Does anyone have any insight on why this would happen.

 

[alldodge]

While starting a new thread may get more eyes looking, the problem is the stuff which lead up to the initial start issue is not for those to read.

You didn't have fuel at the rial and now you do but it won't start or run correctly.

Start by setting the distributor as specified in the manual for a HVS

 

[tpenfield]

Subscribing in . . .

There is another recent thread about a 5.0 MPI No start. Is this the same engine or is there a Merc 5.0 MPI pandemic in the making :noidea:

This is the re-build on the 5.0 MPI :thumb:

"loud bang" as in a backfire?

If you open the throttle before cranking, the engine starts and runs properly? (or does it not run well even after starting?) How much open does it need/like in order to start?

I assume you have checked, doubled checked, and re-checked all of the basics . . . #1 TDC, spark plug wiring, timing is in the ball-park?

 

[aggiekid]

Subscribing in . . .

There is another recent thread about a 5.0 MPI No start. Is this the same engine or is there a Merc 5.0 MPI pandemic in the making :noidea:

This is the re-build on the 5.0 MPI :thumb:

"loud bang" as in a backfire?

If you open the throttle before cranking, the engine starts and runs properly? (or does it not run well even after starting?) How much open does it need/like in order to start?

I assume you have checked, doubled checked, and re-checked all of the basics . . . #1 TDC, spark plug wiring, timing is in the ball-park?

[/QUOTE
Yes, like a backfire. The engine does run fine for a bit with the throttle open.I's say it keep it open about 10 degree, anymore than that it stalls. I did check TDC, spark plug and timing. Although I am not sure if my timing was based at TDC at compression or exhaust. I failed to check TDC at compression stroke. With the engine assembly, I am struggling to rotate the engine to find TDC compression.

 

[alldodge]

Best way to set TDC with the HVS distributor is by building the tool starting on page 15

View attachment Bul 2011 11R2 HVS clocking.pdf

 

[aggiekid]

Best way to set TDC with the HVS distributor is by building the tool starting on page 15

Thank you. I will built this kit, follow the process. I didn't mention previously, but the backfire is through the throttle body. ( dont know if that matters).

 

[alldodge]

With having to open the throttle could mean the IAC is not allowing air in. To see if its working, remove and then reconnect the cable to it. Turn key ON and the tip should retract. Can also clean the tip and bore at this time

The backfiring is from it being out of time or distributor cap is tracking inside. The caps are a known problem with these distributors

 

[tpenfield]

Yes, like a backfire. The engine does run fine for a bit with the throttle open.I's say it keep it open about 10 degree, anymore than that it stalls. I did check TDC, spark plug and timing. Although I am not sure if my timing was based at TDC at compression or exhaust. I failed to check TDC at compression stroke. With the engine assembly, I am struggling to rotate the engine to find TDC compression.

TDC at the exhaust stroke would be of no value and certainly not something you should time the engine to . . .

You might have to get an adapter for the harmonic balancer in order to get a breaker bar to rotate the engine. Rotating via the crankshaft bolt can lead to over tightening and potential damage to the crank.

Assuming you do get a breaker bar set up, with your finger in the #1 spark plug hole (front left (port) side of engine), the compression stroke will be pretty obvious as air will be pushing your finger out of the hole. You will feel nothing on the exhaust stroke. Once on the compression stroke, you stop at the '0' mark of the timing indicator for TDC. (FWIW - my engines just have the actual base timing mark ( 8˚ BTDC) and nothing else, so I have to guesstimate TDC from that mark.)

From what you described, I'd go back through the timing, re-set the distributor position, check proper order of the spark plug wiring, etc . . . .

 

[aggiekid]

Well, this is turning into a disaster. With the intention of trying to find compression stroke, I removed the spark plugs, and cranked the engine. I could not sense air blowing out of the spark plug hole, so I decided to do a compression test. I get Zero compression on cylinder 1, 3, 5 and 7. I get decent readings on cylinder 2,4,6 and 8. Any ideas on what could cause the odd cylinders to have zero compression ?

 

[Bt Doctur]

valves too tight

 

[alldodge]

valves too tight

Agree that would do it
Valves are adjusted 1 full turn after zero lash
Zero lash is the up/down movement, bot if it can be rotated

 

[aggiekid]

Thanks guys. Adjusted the valves, and I now have compression

 

[tpenfield]

Thanks guys. Adjusted the valves, and I now have compression

:thumb:

Now you can go about checking the timing, wiring, etc. BTW - to find TDC compression on #1 . . . just take out the spark plug for #1 and use your finger to feel the air coming out as you rotate the engine.

 

[aggiekid]

Thank you all for the help. After almost 2 months of working on rebuilding the engine, I was able to get it started, and take it to the lake. Appreciate everyone's input.