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99 mercruiser 4.3 mefi3 runs VERY rich and backfire/no power out of the hole

P

Pepind

New member
Any ideas on why it could be running so rich and backfiring. TBI coolant temp sensor has been changed and TPS on order.

Thanks in advance
 
History:
Original motor was a mefi1, block was cracked due to winterization from previous owner. New motor is mefi3 with TBI. I refurbed boat over the winter. Boat ran ok for a few weeks. There was some intermittent surging at no wake speed but shot out of the hole well. Then Motor started to run rich.
Only had a 30 psi gauge for fuel press test and the needle was pinned. Tested fuel press reg and diaphragm lifts at 30psi. Light kink in return hose restricted flow. Changed hose. Now a steady 28-30psi pressure.
The surging disappeared but still runs rich and now I get backfire at TB and bog down when punching it to get on plane. Feathering the throttle up slow will sometimes get me there but other times the same symptoms occur.
Following codes were present
14 ECT low temp.. New one installed
22TPS sig.voltage low
23MAT low
34MAP low
ive swapped all these parts with parts from old motor as well as TB/ injectors... Codes were cleared by a local mechanic. but no change in symptoms on water or on muffs. VERY rich and backfire/loss of power.

Ive just skimmed through the service manual to get an idea for a path forward on troubleshooting.
If anyone has an idea on where to focus I'm all ears.
 
Additional info:
Compression tested good, it was checked with a reader by the mechanic, he changed the temp sensor and has new TPS on order. However, I've been reading codes with a jumper and continuity on my meter through the jack.

also it did have some old/ water gas. has new ethanol free gas, went through entire fuel system, I had tank out and cleaned blew out hoses. Filter change. Trying to go from memory as my notes are with the boat at my father I laws shop.
 
Last edited:
Being as all of the codes you listed are saying LOW readings present, I'd think you have a reference voltage issue for those sensors....and its highly unlikely that ALL of them are bad. I'd start with the connectors at the ECU and the harnesses with them.....
 
Update

unplugged all sensors with codes, at connectors, all have 12v between black ground wire and battery positive. All have 5 volts between reference and ground. Plugged them back in and function tested sensors by slipping a small needle in parallel to wire until I touched the terminal in plug. With engine off I confirmed continuity from needle to proper pin in J plug at ECM.

TPS voltage was low and no change as throttle was applied. Both throttle sensors I had were bad. I believe I damaged one a week ago turning it the wrong way when checking resistance. We changed the TPS with one pulled from a scrap yard. Good. Voltage is 0.5v to 5v linear with throttle. Seems to have cleared the running rich issue. ECM had no throttle position reference so it seems it just fed the motor a known value of fuel as a limp home?

MAP sensor was functioned tested with vacuum. Voltage is linear from 5v down to 0.5v as vacuum is applied. Good.

only code left was the MAT

MAT sensor is a 2 wire sensor. I can't read resistance across thermistor. I believe it's open. There is 5.17v across disconnected plug, when connected there is still 5.17v even when heat is applied with a heat gun. Definitely open. Ordered new one.
this sensor is mounted to the flame arrestor which gets carboned up at Throttle body backfires and probably fouled/burnt the thermistor out. I will set the new one near the flame arrestor so as not to fry or burn it out until confident it doesn't backfire anymore. Picking part this morning.
 
Was the "new" motor actually new or used. If it is a used motor it might be a good idea to remove the valve covers and check for proper valve lift. A worn camshaft can cause problems similar to those you describe. When you pulled the plugs to do the compression test, was the color of the electrodes consistent?
 
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