1999 4.3 vortec mercruiser on a 1900sr maxum

Chris60

I have replaced and inspected all electrical and fuel components including the motor. It did not correct the original symptom. which is when I get up to 3000 rpm it starts popping and pining. Is there something I am over looking please advise. In addition this boat was in the shop for 6months and the pros could not figure it out.  

PickleRick

Carb or efi?

Chris60

This is a throttle body. The EFI was replaced and the pressure going to the throttle body was checked ok. 

aero3113

How about bad gas or a pinched fuel or vent line? Does it seem to be straining also? Could it be your gimbal bearing? Being that you replaced the engine, there's not much left. Could be a bad ground somewhere also.

PickleRick

Have you checked for spark on each cylinder? Read plugs on each cylinder? Compression test?  

Does this have a coil pack? 

Chris60

All compression gas and plugs were all tested. Takes right off no sluggish until its under a load and gets up to 3000 rpm than pining and popping starts. 

PickleRick

You should be able to use a scanner to determine which cylinder(s) are missing.  Could be a simple 2 plug wires crossed?  Double check firing order. 

Could also be injector(s) 

Have the tps and map sensors been bench tested?  Both cold and after being heated up with a heat gun or hair dryer

PickleRick

If you cant use a scanner it may be possible to run a digital thermometer on each runner of the manifold and try to find cooler running cylinder?  

Question Alswag, on a twin engine i can find a weal cylinder by removing one plug wire at a time. If one is weak there will be little to no change when i remove the weak cylinder plug wire. To find a weak cylinder on a v6/v8 could someone remove a plug wire and then cap the dizzy cap lead with an insulator one cyl at a time, accelerate boat to plane and if all but 1 or  2 make a drastic change in power you found your guilty party? Would it harm the motor?

The reason i ask is raw water cooled motors dont run a good even temp so using a thermal test might give false results. 

PickleRick

Ok thanks. And if i did this id cap the lead on the distributor cap to prevent live spark. Even then an open and well ventilated hatch should always be a priority when playing with spark or fuel.

Chris60

Chris60 said:

All compression gas and plugs were all tested. Takes right off no sluggish until its under a load and gets up to 3000 rpm than pining and popping starts. 

Chris60

This is the same problem I had with the old motor and now with the new motor. So I don't think its the valves. Timing was checked properly also. Could it be a sensor and if so which one would I check. I do see that the oil pressure one goes to the fuel pump.

PickleRick

Map sensor and tps are the first ones id bench test.  I've had a bad tps in a 4.3 s10 that bench tested fine. Random spits/sputters and occasional tps cel led me to check wiring. Wiring checked good. Must have been a dead spot in the tps that was so small my digital multi meter did not pick it up.  An analog mulit meter is at times superior when doing diagnostics.  Anyway, i tried a cheap ebay 12 dollar sensor as it was a cheap risk test, problem solved.  



Also doesn't this have the fuel regulator that was painted is is prone to flake off and clog?  

Are injectors in this engine from the old engine?

Chris60

This fixed my boat. I checked the shift interrupter switch at first thinking that could be it but it shut down the boat correctly. now this is the stumper. I than took my meter and set it to OHMs and when I manually applied the button and wiggled it the readings kept changing now remember this also goes directly to the ECM. Those readings were to far apart. Which will give the ECM the wrong message. So I went out in the water after I made a jumper wire to bypass the switch. Remember when you unplug the switch the motor will shut off. Put the jumper wire that is shaped into a u shape into the ECM side of the plug. Re-fire the boat and very carefully put it in gear and open it up and see if that corrected the problem. I did all of that and my boat runs like it just came out of the factory. I ordered a new switch today. Hope this helps someone before they spend a couple thousand and a switch is around $40. 

PickleRick

Good find! 

Chris60

PickleRick said:

Good find!  Thank you!