99 7.4L MPI will not rev up, runs smooth.

Frank I have forwarded your note to my mechanic on timing. I will keep you posted and hopefully we can all learn something from this.
 
You didn't drop an exhaust shutter into the "Y" pipe when the motor was installed ??? A blocked exhaust will kill the RPM's.
Once found this, half the shutter in behind the prop and the other half stuck in the exhaust bellows.

BTW: If you have fuel pressure you have the volume, fluid dynamics ! If the injectors are fully open (wide pulse width) and you are still maintaining 35psi on the fuel rail, the pump is certinally giving you the volume.
 
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Adamo said:
Hi Capz,

I boat in the Pacific Northwest so I don't think it is vapour lock issue. I must say I wondered about a volume test instead of just a pressure test. The vent line seems to be fine but I have not checked the anti-syiphon valve. I just discovered that this is a potential problem but I am not sure how to check/repair this item and why exactly this would be a problem.IE how it would show good pressure yet provide poor volume??
Click to expand...

Pressure and volume are related. IF the pressure is correct, then assuming the injectors are clean and firing for the required duration, then the volume will be correct. Simple physics. If the flow to the fuel rail is restricted then the firing of the injectors will reduce the pressure in the fuel rail. If your pressure is correct, then you do not need to look at the upstream components of the EFI system. Only components downstream of the pressure tap are suspect. Since the only components downstream of the pressure tap is the rail and injectors, your mechanic did the right thing by testing the injectors.

Good point on the restricted exhaust system. I had a car with a 440 that acted like it had a couple of japanese squirrels under the hood. Turned out that the inner wall of the exhaust pipe de-laminated and blocked all but 1/2" of the 3" pipe. On the other hand, my previous boat with a stern drive lost it's shutter, which didn't affect performance at all. Found it at the end of the season when I pulled the drive. Odds are, the tech would have found a problem with that when he pulled the engine for the rebuild. Still, worth checking.

Best regards,
Frank

Best regards,
Frank
 
Pressure and volume are related. IF the pressure is correct, then assuming the injectors are clean and firing for the required duration, then the volume will be correct
Click to expand...

But the fuel pressure is showing 35psi at lower rpms. Let's say there's a crimp in the line, the fuel pump was failing or the anti-syphon valve was partially closed would you absolutely see a pressure drop as the engine can't rev up with inadequate volume? I'm not arguing the point. I'm just throwing things out there to understand the system better as I have the same engine. Note I only have a few years looking at boat engines.
 
Hi guys, Spoke to the mechanic today and he tells me these are the worst injectors he has seen in 35 years upon dismantling them. He feels we have found the problem. I will know more in about a week when everything arrives and gets installed. I am unsure if the cool fuel system and injector corrosion could have caused the engine failure last year. After a two hour run the boat sat for a day or so and then immediatly on restarting the engine the piston rod on the fourth cylinder blasted through the top of the piston and the motor was seized. I have been told this was due to water in the cylinder. What I don't know is how the water got there. What I have been told is that the amount of water needed to cause this damage could not have come thru the fuel system and that it is much more likley water coming into the cylinder thu the exhaust. I am not sure how this is possible and if that Exhaust Shutter you are asking about could be related to this. What kind of a system leaves an engine open to seawater? What is supposed to stop this?
 
I have followed this thread daily and I must say that I now have a greater appreciation for injectors and how they work. Great information here! On a side note; a friend forgot to cover the 6" exhaust on a 6/71 last fall on his work boat and water filled one cycl; started it up this spring and I could not believe you could bend a rod the size of my wrist.
 
Hello Gentlemen,

Unbelievabe as it seems to me we have changed the cool fuel system/fuel pump and all eight injectors and spark plugs and the problem is still here. The shop tells me these items were in very poor shape and needed to be changed but they have not solved the problems. HELP>>>
 
I had a similar problem with my 97 7.4L MPI. Ended up being a timing sensor in the distributer. Wouldn't go past 3200rpm.

Good luck
 
I would print out fc3's post on page 2 and hand it to your mechanic before he starts pulling out the stringers.....
 
Swap distributors from the port and starboard engines?
 
Thanks guys. I really appreciate your time and thoughts. We will be testing the timing sensor tomorrow and I have sent along FC3s note from page two. I will keep you all posted. I am thinking this is electrical at this point and if the above tests don't resolve this problem I want to look for correct voltages at various critical electrical components around the engine. We disconnected the tach yesterday to see if a faulty tach was to blame.

Cheers for now.
 
Last question for tonight. Anyone tried the RACOR filter/water separators with the glass bowl at the bottom to allow inspection visually. I would like to try this in place of a Sierra filter I am using now.
 
I was thinking the ecm is going into a fail safe mode and limiting rpm, but that would store a readable code. And you said no codes came up. But what if a sensor is giving false readings to the ecm? Instead of interpreting that as a fault, it will adjust spark and fuel delivery based on the false reading. For instance, the TPS sensor should read .5 volts at idle, 4.5 at wot. If it's shorted, a trouble code is set. But if the circuit is giving false readings, the ecm won't deliver the right amount of fuel because it thinks the throttle plate is at a different angle than it really is. After checking timing, maybe a test of individual sensors could be necessary.
 
Had it happen on a friends boat last year.....restricted exhaust. The water restrictor broke off and found its way into the exhaust boot.Put a vacuum gauge on the motor see if the intake builds pressure as you increase throttle opening.
 
Today we found the pin J2-6 (Ignition Control Ref. Low) on the harness that plugs into the distibutor to have a dead short. The grounding out stopped when the rest of the harness was unpluigged from the ECM, a Delphi Part. We therefore pulled the ECM for testing. I will keep you posted. I think the problem is in the ECM although no error codes or in the timing sensor.​
 
Hi gentleman,

You have been helpful in posting to my threads thus far and I am wondering if you can offer any thoughts on this. I have confirmed my 99 searay 7.4MPI has a MEFI3 ECM but it also has knock sensors in the engine not in the ECM which is what I read would be normal. Is it possible the engine does not match the ECM? I know the ECM was changed on delivery and they install part number 861720T 1 and it ran fine says the dealer doing the PDI. This correlates to the 16237009 DELPHI number on the unit I now have. I am having the typical 2800rpm drop off to safe mode problem and cannot resolve the problem yet. My mechanic has found what he believes to be a dead short as a result of testing for ground in the harness at the distributor in CKT 456 PIN (J2-6) which correlates to (Distributor Reference "low").
He determined from reading the manuals that this harness is to ground at the distributor but it is still grounded in that circuit only when the the harness is removed from the distributor. If he disconnects the ECM (MEFI3) from the harness the ground is gone. He is therefore surmissing that it is the ECM that is grounding this out in error and this is what is causing the drop in RPM at the 2800 "safe mode" level. I feel personally that this is a best guess on his part and that it is more likely a faulty sensor(which one??). Unfortunatly we get no error codes to give us any clues. We see that the timing will not advance and that is about it and it starts to struggle right at 2800RPM and on our last test run we got an alarm for the first time at around 3200RPM and the engine, just prior to the alarm, dropped back on it own to 2800.

Long note. Sorry but detail is everything with these sorts of problems.

Adamo
 
Here's how I'm understanding this. Scenario 1: The ECM is not detecting voltage from the knock sensor and goes into safe mode to protect the engine. Scenario 2: The ECM voltage monitor is continuously receiving a signal from a bad knock sensor and retards timing.
Both situations would result in an rpm drop. The difference is a code will be set only with scenario 1. In Scenario 2 the ECM thinks all is well and does what it's supposed to, so no code.

Does the ECM match the engine? If your original engine had MEFI1 or 2 then you also have a module which shuts off the voltage from the knock sensor to the ECM. This tells the ECM that a knock is present.
Now let's say you throw on a MEFI3, which isn't designed to work with the knock sensor voltage module. MEFI3 works by detecting voltage directly from the knock sensor. This looks like it could be a major conflict, assuming you have the knock sensor module working in conjunction with the sensor. So with the module continuously sending voltage to the MEFI3 ECM (indicating no knock), the ECM would interpret that as a knock and retard timing. MEFI1&2 works completely opposite from MEFI3, regarding the knock sensor. MEFI1&2, no voltage to ECM indicates a knock. MEFI3, a knock is present when it receives voltage.
 
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Adamo said:
Today we found the pin J2-6 (Ignition Control Ref. Low) on the harness that plugs into the distibutor to have a dead short. The grounding out stopped when the rest of the harness was unpluigged from the ECM, a Delphi Part. We therefore pulled the ECM for testing. I will keep you posted. I think the problem is in the ECM although no error codes or in the timing sensor.​
Click to expand...

J2-6 is the pin for Ignition Control Ref. Low on MEFI-1 and -2. It's J2-3 on MEFI-3. It's supposed to be 0 (less than .5) volts and held there by the PCM. J2-6 is not used on MEFI-3.

As above the knock sensor module is integral to MEFI-3. It's external on MEFI-1 & -2. The L-29s use J1-30 and J1-14 to directly connect the knock sensors to MEFI-3. If the ignition wires to the plugs run too close to the knock sensor wires, MEFI can interpret the cross-talk as knock and retard the spark. Use the scan tool to read the number of degrees that the spark is retarded. Should be zero.

Forgot you have a sterndrive. J2-24 is the input from the sterndrive lube bottle. Make sure this circuit (906 tan/white) is not grounded.
 
Capz,

A little closer reading and I have determined that I have a MEFI 3 and there is knock Sensors in the engine it is just that the module is in the ECM on MEFI 3. All is good there. On our last test run we disconnected the knock sensors just to see what might happen and we got the same poor performance only this time we did get an alarm. The DDT was not hooked up so no idea of trouble codes or not. We also disconnected the alarm on the drive oil reservoir.

I am wondering if the plug wires being too close to the sensors could be it. We had this problem before the rebuild but it was very intermittent. Now it is permanant. I will be checking this. Would be too good to be true but we are getting desperate.

I am also wondering if the plug wires failing somewhere could cause my trouble. From the readings it appears the timing not advancing is the issue. The question is what is stopping the timing from advancing. Could it be plug wires?? Bad sensors yet no codes to suggest this.

Thanks all,
Adamo
 
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