4.3 MPI MAP/MAT problem?

[Mischief Managed]

I'm helping a friend with a 2003 4.3 MPI engine issue and looking for a little advice. The problem started last year with a faulty IAC. I replaced that and the engine ran perfectly for awhile but then started intermittently losing power on-plane and giving a 2 beep warning this year. When it was failing under load, it would operate perfectly then suddenly lose power and die. The engine has also become increasingly difficult to start and at one point would only start if you give it some throttle and even then it was a 50/50 shot at best. Now it will fire with the shifter in neutral (idle speed), but will not stay running and I cannot even get it to run with the throttle open. I tested the fuel pressure and it was reading 40 PSI with the key on. It will slowly drain down to 30 PSI, and then it holds (guessing a leaky injector, but I don't think it's the main problem). I checked the vacuum fuel pressure regulator, and it does not leak, but I have no idea it it really works or not. My gut says it's fine. I swapped out the throttle position sensor since it's very inexpensive and easy to do. The crankshaft position sensor connector looks clean and corrosion free. I tested the coolant temperature sender resistance vs. temperature and it reads normal (the temperature instrument reads is fine too). The MAP/MAT connector is clean and corrosion free. The IAC moves smoothly. The boat owner is trying to get mechanic with a scan tool to look at it since it's surely stored some codes, but it's a terrible time of year to find an available marine mechanic around here. Since the IAC and TPS are good, and the ignition fires normally, I suspect it's the MAP/MAT sensor. Is there any test I can do to it without the engine running? I took it out and inspected it, and it looks fine. The MAP/MAT sensor is over 200 bucks so I don't want to recommend swapping it until I can prove it's bad. Any other ideas?

 

[alldodge]

I'm helping a friend with a 2003 4.3 MPI engine issue and looking for a little advice. The problem started last year with a faulty IAC. I replaced that and the engine ran perfectly for awhile but then started intermittently losing power on-plane and giving a 2 beep warning this year. When it was failing under load, it would operate perfectly then suddenly lose power and die. The engine has also become increasingly difficult to start and at one point would only start if you give it some throttle and even then it was a 50/50 shot at best. Now it will fire with the shifter in neutral (idle speed), but will not stay running and I cannot even get it to run with the throttle open. I tested the fuel pressure and it was reading 40 PSI with the key on. It will slowly drain down to 30 PSI, and then it holds (guessing a leaky injector, but I don't think it's the main problem). I checked the vacuum fuel pressure regulator, and it does not leak, but I have no idea it it really works or not. My gut says it's fine. I swapped out the throttle position sensor since it's very inexpensive and easy to do. The crankshaft position sensor connector looks clean and corrosion free. I tested the coolant temperature sender resistance vs. temperature and it reads normal (the temperature instrument reads is fine too). The MAP/MAT connector is clean and corrosion free. The IAC moves smoothly. The boat owner is trying to get mechanic with a scan tool to look at it since it's surely stored some codes, but it's a terrible time of year to find an available marine mechanic around here. Since the IAC and TPS are good, and the ignition fires normally, I suspect it's the MAP/MAT sensor. Is there any test I can do to it without the engine running? I took it out and inspected it, and it looks fine. The MAP/MAT sensor is over 200 bucks so I don't want to recommend swapping it until I can prove it's bad. Any other ideas?

Does this engine have the crab (crap) style distributer cap? What's the serial number?

 

[Mischief Managed]

It does have the crab style cap, no idea on the serial number. It does have a ECM555, if that helps. The boat owner is thinking hard about buying a merc scan tool. The engine is literally dying to tell us what's wrong...

 

[Tail_Gunner]

30 psi will not run a mpi engine 30 lbs is tbi territory i think 45 for mpi. Red lean burn knocking..sensors detect knocking and retard timing its a endless loop of error's. Remeber this is not a carb it can get really strange when things get lean.

 

[Mischief Managed]

It drops to 30 psi when it's not running, when the pump first turns on, it jumps to >40. Was not able to test fuel pressure when it was running because I can no longer get it to start.

 

[alldodge]

Reading though the 555 manual No 36 you fuel pressure is good enough to start. Pressure should be above 35 psi and can drop but does not drop to 0. Check for spark and if that is good, next do a compression test.

Have you tried swapping the Main power relay with the fuel pump relay?

Check the ECT sender

 

[Tail_Gunner]

It drops to 30 psi when it's not running, when the pump first turns on, it jumps to >40. Was not able to test fuel pressure when it was running because I can no longer get it to start.

My bad quick read, do you know how to use a vacum gauge and check the electrical values of your map sensor that would be the first order of business

Good file it has much more info than it looks like take a very close look

https://drive.google.com/file/d/0B3waQufF7aAAQnVaNmlWQUI0Z2s/edit?pli=1

 

[Mischief Managed]

The problem was found and fixed. The owner purchased and hooked up a Tech Mate scanner to the engine and it did not power up. He contacted me and I worked remotely with him(I was on a business trip) I had him test the power (purple wire to pin D) and ground (black wire to pin A) wires from the 4 wire diagnostic connector and the ground was good, but the +12 volts was not present. Traced (logically with schematics, not physically) the purple wire all the way back to the ignition switch and found that the female spade connector was loose on the back of the switch. Tightened it up and the boat is running perfectly. There are no codes stored in the ECM, it was simply not getting reliable power to pin B18 on the ECM 555. The boat owner was so pleased, he's giving me the Tech Mate scanner for my help (between driving and working on the problem locally, and working on the problem remotely, I have about 10 hours invested in this problem...).