power loss - only at 3/4 throttle

[00Challenger2K]

Question for the 240EFI experts:
I bought a 2000 Challenger 2000 last November that had some issues with the 240EFI, M2 package. Engine issues at the time were, no spark in 2 cylinders. So, I replaced the trigger module. That resulted in getting spark at all 6 plugs!
After running the witches brew through the engine & performing the prescribed winterization steps, I then replaced all of the spark plugs, the water separating fuel filter (and sensor), the inline fuel filter by the small electric fuel pump, and the port side head temperature sensor. I also completely drained the fuel tank and oil tanks (main & reserve) and let them remain empty over the winter. This spring, I filled up the respective tanks up with fresh (non-ethanol) fuel and Mercury 92-858027Q01 oil. After re-installing the battery and changing the lube in the jet pump, the boat started up just fine. It idles great and runs great at all speeds - well...just about all speeds. Here is the issue:
If view the throttle with 8 imaginary positions with position 0 being idle and position 8 being WOT, I am experiencing a problem at throttle positions 6-7. If I'm running at WOT about 6000rpm and 55MPH, and I back down the throttle just a little to the 7 or 6 position, the engine drops down to like 2000 rpm and the boat comes off plane. The engine sputters and surges if you leave the throttle at this position. It does not die. Just runs oddly with much lower rpm. Now, if I pull the throttle back to the 5 position, engine rpm peaks up again (like about 4000rpm) and the boat gets back up on plane and runs great.
This situation happens if I am coming from throttle position 8 downward to 7-6 but it also happens if I ramp up slowly from position 0. Positions 1, 2, 3,4, & 5 all work as expected....Engine RPMs rise and the boat goes faster and faster. UNTIL you get to position 6-7. There, RPM's drop to like 2000 again and boat slows down and surges. But pushing past that to position 8 hits WOT again and RPM's rise to ~6000 again and boat runs great moving along at ~55MPH.
So, what's going on? What engine component(s) could cause this? I already replaced all of the common-cause and tune-up components, so I'm baffled. Could this be an adjustment issue...like timing adjustment or TPS adjustment?

I do have the mercury service manual as well as the manual used to train Mercury service techs on the 240 EFI engine, but I don't have access to the expensive Mercury diagnostic plug-in tools, so I'm hoping someone here can shed some light on what may be going on in my situation.
Thanks, in advance to anyone who can help me figure this issue out.

 

[00Challenger2K]

BTW - Engine S/N = 0E385325

 

[Dr Honda]

That's a new one on me. But... off the top of my head... I'm going to say you have a bad TPS. (Throttle position sensor) More or less... it's the only thing that tells the computer where your throttle is at. AND... since it sits at the 65%~75% throttle most of it's life... that's where the problem will happen.

Since you have the service manual... it's outlined on how to check it. The only down side is... the early TPS is crazy expensive. (about $500) We spent a couple years... as a group looking for a suitable replacement since it should be a $60 part. The issue was... most automotive units were 6K ohm, and this one was 10k ohm.


The only other thing that comes to mind... and it will be cheap to fix if you find this... the trigger coil physically moves. it's connected to the throttle linkage. since it moves, and it's behind the flywheel that is also moving... maybe the wires have gotten chaffed, and they only touch (to ground) at that throttle position. (taping up, and moving wires is free) Even if you find that the wires have broken, or a crack in the trigger unit... it's still a lot cheaper than the TPS.


Good luck. let us know if you find an issue with either of those.

 

[00Challenger2K]

Thanks Dr. Honda!
I was also suspecting the TPS, but was hoping that I was missing something else because I know those ones for the early models are quite expensive and I'm not even sure where to purchase a replacement one from. I will follow the procedure to check the TPS and see what that results in. First though, I will inspect the wiring going to the trigger module - just to make sure the issue isn't in that area.

One related question: I have read some articles that seem to indicate a bad TPS usually results in hard starting and excessive smoking. Is that true? Since I have neither of those issues, I was starting to think it may not be the TPS. Your point about it spending most of its life at 65%~75% is a great point though. It makes sense that this is where it would wear out.

Also, could this sort of problem stem from anything in that big metal fuel vapor separator assembly that's somewhat under the small oil reservoir tank on the front of the engine? That is the only other component that I haven't done anything with, and don't really know much about. Just curious.

Thanks again!!

 

[Dr Honda]

highly doubt it's in the vapor separator. It's basically a "Toilet Tank" with a fuel pump. SO... fuel goes in, when it's full, the float physically pushes a valve closed, and fuel stops. (The pulse pump, and/or electric fuel pump feed it under low pressure) The high pressure pump feeds the fuel injector rail, and what fuel doesn't get used simply returns to the tank. (The pressure regulator is in the end of the rail) Sure... you could get intermittent issues... but since it's not really computer controlled... the throttle position would not effect it.


TPS: Yes... without a doubt, it can cause a lot of other issues. But only if it shorts out, or goes open. In this engine... the computer controls the fuel with the RPM it sees. The TPS really only trims the fuel. SO... at wide open throttle... the computer stops looking at it and gives full fuel. At idle... it's looking at RPM, and the TPS (and the port side temp sender) to know how much fuel it needs. When the TPS goes open, or is giving the computer an "out of range" reading for what it's expecting... it assumes it's bad, and defaults to full fuel for the given RPM. At that point... it over fuels, and causes hard starting, and smoke... and so on. If there's just a dead spot in the wiper (and it happens) then you could get an OK reading low, but eventually go out of range. but then, at full throttle... it doesn't mater.


It generally wouldn't fuel so hard that it would drop way down to 2000 rpm... but the symptoms are what you describe. (Clean, clean, bad, clean)

It's an easy test to do, and......................................... actually.......

OK... the book wants you to look at voltage, and you need a harness to check it. Just do this.... Open the 3 pin plug on the end of the TPS, and grab an ohm meter. Put one lead on the center pin, and the other on either side. Have a helper slowly move it from low to high throttle. (maybe a couple times) and just watch the reading. It should be a smooth continuous change. If you see a jump, or a drop (a non-linear reading)... then it's gone bad.

FYI... if you move from one outside pin to the other... the reading will just change direction. (High to low, vs. low to high)