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MM230
06-09-2012, 04:58 PM
I have a 99 Maristar with the 5.7 LTR/fuel injected/northstar ignition. It sat for 7 years with a bad motor before I picked it up. I purchased and installed a new long block. I installed the existing intake manifold, oil pan, etc. When I start it up, it revs up, then comes back to idle speed and stalls. I replaced the fuel pump with a new one from MC. I also replaced the MAP sensor. Funny thing is, if I start it with the MAP sensor not installed in the intake and my finger over the hole where the sensor goes, it will run and idle. I can even quickly install the sensor while it is running and it will continue to run. It seems to run lean though. But after shutting it off and trying to start again, I get the same problem. Revs up, comes back to idle speed and stalls. Anyone run into this problem before???

MM230

AFX-30
06-09-2012, 05:02 PM
Did you onhook the batery after installing the fuel pump to reset the ECM. I had this problem after installing my fuel pump. Unhooked the battery for 5 mins and then it ran like a champ.

MM230
06-09-2012, 07:57 PM
tried that. I'm wondering if my ECM is bad. I found that if I unplug the temp sensor, it will start and run fine for about 3-4 minutes and then it just shuts off. I can start it right back up and it will do the same thing, run for 3-4 minutes just fine, even able to rev it up to higher rpms. then it dies after 3-4 minutes. I replaced the temp sensor and get the same results. This really has me baffled. I may just have to take it to the MC dealer and have them diagnose it.

JimN
06-09-2012, 08:30 PM
I have a 99 Maristar with the 5.7 LTR/fuel injected/northstar ignition. It sat for 7 years with a bad motor before I picked it up. I purchased and installed a new long block. I installed the existing intake manifold, oil pan, etc. When I start it up, it revs up, then comes back to idle speed and stalls. I replaced the fuel pump with a new one from MC. I also replaced the MAP sensor. Funny thing is, if I start it with the MAP sensor not installed in the intake and my finger over the hole where the sensor goes, it will run and idle. I can even quickly install the sensor while it is running and it will continue to run. It seems to run lean though. But after shutting it off and trying to start again, I get the same problem. Revs up, comes back to idle speed and stalls. Anyone run into this problem before???

MM230

You need to check for codes and if possible, take it to a dealer for diagnostics. That will give you a baseline about whether the sensors are within range, if the engine's ground is connected well, if it's running at the correct temperature, etc.

You also need to check fuel pressure, do a compression test and check cylinder leak-down. If you have a problem with vacuum, it will show up then.

It's possible that you have a problem with the IAC or the IAC driver in the ECM. Diagnostics will determine this, too. The diagnostic computer has a way to operate the IAC for raising/lowering RPM. If it works well, it should run properly.

Look at the throttle plate- if it doesn't have a hole in it (should be about 3/16" diameter), the throttle body may be from a 2000 model and the ECM may be from a '99. Contact MC and give them the serial number of the ECM and the engine, if you have it. If not, give them the hull ID number and they can tell you if the ECM matches the boat. If it doesn't, it may have been re-powered before you got it.

MM230
06-11-2012, 12:40 PM
You need to check for codes and if possible, take it to a dealer for diagnostics. That will give you a baseline about whether the sensors are within range, if the engine's ground is connected well, if it's running at the correct temperature, etc.

You also need to check fuel pressure, do a compression test and check cylinder leak-down. If you have a problem with vacuum, it will show up then.

It's possible that you have a problem with the IAC or the IAC driver in the ECM. Diagnostics will determine this, too. The diagnostic computer has a way to operate the IAC for raising/lowering RPM. If it works well, it should run properly.

Look at the throttle plate- if it doesn't have a hole in it (should be about 3/16" diameter), the throttle body may be from a 2000 model and the ECM may be from a '99. Contact MC and give them the serial number of the ECM and the engine, if you have it. If not, give them the hull ID number and they can tell you if the ECM matches the boat. If it doesn't, it may have been re-powered before you got it.
Thanks for the advice JimN. I verified that the ECM was for this boat. Fuel pressure is constant 41psi at the schrader valve. Compression on each cylinder is good. I was able to get a scan tool to read the codes. The only codes I get are 14 and 34. I understand that 14 is ECT voltage too low and 34 is MAP voltage too low. I unplugged both of these while the engine was running so I would suspect these codes are because I unplugged the sensors and not because they were failing. Both are new parts. I should add that my problem seems to be just at idle. I can actually rev up the engine and keep it running, but when I return the throttle to the neutral position it won't idle but shuts off instead. Do I need to reset these codes for the engine to run properly?

The next thing I was going to check is the injectors. I'm thinking that after sitting for 7 years they may have become clogged.

MM230
06-11-2012, 05:59 PM
Thanks for the advice JimN. I verified that the ECM was for this boat. Fuel pressure is constant 41psi at the schrader valve. Compression on each cylinder is good. I was able to get a scan tool to read the codes. The only codes I get are 14 and 34. I understand that 14 is ECT voltage too low and 34 is MAP voltage too low. I unplugged both of these while the engine was running so I would suspect these codes are because I unplugged the sensors and not because they were failing. Both are new parts. I should add that my problem seems to be just at idle. I can actually rev up the engine and keep it running, but when I return the throttle to the neutral position it won't idle but shuts off instead. Do I need to reset these codes for the engine to run properly?

The next thing I was going to check is the injectors. I'm thinking that after sitting for 7 years they may have become clogged.


I just thought of something that I thought I'd ask about. I did replace the original motor with a rebuild/bored long block. Would the new bore .050 be causing this problem? Is it possible something needs to be changed in the computer to accommodate the new bore?

MM230
06-11-2012, 07:11 PM
I've been messing with this thing all day trying different things. I can spray a shot of carb cleaner in the intake just before it stalls to keep it running. While doing this it seems to hunt up and down for the correct rpm. After about 5 tries without the motor completely stalling it will continue to idle properly and very smoothly without hunting up and down. RPM is constant. After several minutes it will lean out and die. It won't continue to idle. At this point I've replaced the MAP sensor, TPS,IAC, and Temp sensor.

JimN
06-11-2012, 07:34 PM
Thanks for the advice JimN. I verified that the ECM was for this boat. Fuel pressure is constant 41psi at the schrader valve. Compression on each cylinder is good. I was able to get a scan tool to read the codes. The only codes I get are 14 and 34. I understand that 14 is ECT voltage too low and 34 is MAP voltage too low. I unplugged both of these while the engine was running so I would suspect these codes are because I unplugged the sensors and not because they were failing. Both are new parts. I should add that my problem seems to be just at idle. I can actually rev up the engine and keep it running, but when I return the throttle to the neutral position it won't idle but shuts off instead. Do I need to reset these codes for the engine to run properly?

The next thing I was going to check is the injectors. I'm thinking that after sitting for 7 years they may have become clogged.

The code scanner may have a reset button on it- look for that.

If you have a multi-meter, measure the resistance of the ECT when it's at ambient temperature and when it's hot. You can do this by heating some water on your stove, in a pan. NEVER apply heat directly to the ECT. If you read more than about 10K Ohms when it's hot, you have a bad one- that would be the resistance for about 32F. If you see about 200 Ohms when the engine is cold, the ECT is bad- that would be a good reading for about 200F.

If you look for the tip of the IAC, watch it when you increase RPM and also when you decrease RPM. If it stays closed after reducing RPM, you may have a bad one- it's supposed to close, to allow the RPM to drop faster, then reopen to allow proper idle speed.

JimN
06-11-2012, 07:36 PM
I've been messing with this thing all day trying different things. I can spray a shot of carb cleaner in the intake just before it stalls to keep it running. While doing this it seems to hunt up and down for the correct rpm. After about 5 tries without the motor completely stalling it will continue to idle properly and very smoothly without hunting up and down. RPM is constant. After several minutes it will lean out and die. It won't continue to idle. At this point I've replaced the MAP sensor, TPS,IAC, and Temp sensor.

Stop using the parts cannon. If something is out of range, it can be measured but you need to know the proper values.

Look at the crank sensors. If they're too far from the star wheel, it will cause problems.

MM230
06-11-2012, 08:12 PM
Thanks for your time and all of the advice. I'm at a loss. I've checked all of the things you suggested but I still get the same result. Runs great at higher RPM's, but at lower RPM's it won't stay running. I did all of the ohms testing too before replacing parts. They were within spec, but I replaced them just to be sure because the problem is so frustrating. I can't see the IAC unless I pull it out. When I turn the ignition to the on position, the motor steps, then when I turn if off it steps about 11 times and then returns. Any other tips you could give before I throw in the towel would be appreciated. I don't have a dealer near me to take it to.

JimN
06-11-2012, 08:38 PM
Thanks for your time and all of the advice. I'm at a loss. I've checked all of the things you suggested but I still get the same result. Runs great at higher RPM's, but at lower RPM's it won't stay running. I did all of the ohms testing too before replacing parts. They were within spec, but I replaced them just to be sure because the problem is so frustrating. I can't see the IAC unless I pull it out. When I turn the ignition to the on position, the motor steps, then when I turn if off it steps about 11 times and then returns. Any other tips you could give before I throw in the towel would be appreciated. I don't have a dealer near me to take it to.

The IAC stepping 11 times is exactly the number it should, to open after shutting it down. It should have closed completely on shut-down.

Here's the Cliff Notes for how these work-

The MAP checks barometric pressure as soon as the key is turned on. The ECT tells the ECM the temperature and together, they "know" how much fuel to deliver (pulse width and fuel pressure determine the exact amount).

The TPS needs to be below 2% for the ECM to "want" to idle. The IC (Ignition Control) module tells the ECM the RPM and during cranking, it's usually just below 300 RPM. If the ECM sees more than 300 RPM, it goes into "Run Mode". If the ECM is seeing low RPM, normal MAP & ECT but the TPS shows more than 2%, the ECM is getting conflicting data- <2%, more than 300 RPM but around 650 RPM and normal MAP (high vacuum) makes the ECM think it's above idle speed. The TPS is what tells the ECM it's idling as much as the RPM and vacuum. If the TPS reading is way off, the ECm will default to 12% and the boat will run, but it will not have the best throttle response. If the MAP sensor is sending bad data, there's no default setting and it will definitely not respond well to throttle changes.

All of the sensors can be checked with a multi-meter while the engine is running, but you don't want to pierce the wire's insulation to get to the wire- that opens the insulation and makes it easy for moisture to get in and cause corrosion. This wouldn't be so bad if it corroded only at the spot where the insulation was pierced, but it usually wicks to some other place and that makes it damn hard to find. You want to use a stick pin to back-probe the terminals. The Black wires are all grounded and you should be able to measure less than .1 Ohms from the battery - POST (not cable clamp).

Look for a schematic that was uploaded by EngineNut in the last few weeks- this has the sensors, ECM plugs, pin-out for the wires and other data that you need.

MM230
06-11-2012, 09:40 PM
Thanks again for the reply. I've got to be overlooking something. I'll get the schematic and go through everything step by step.

JimN
06-11-2012, 09:50 PM
Thanks again for the reply. I've got to be overlooking something. I'll get the schematic and go through everything step by step.

Look at the idle stop on the throttle body- if the screw looks like it could be adjusted because the paint is gone, it's possible that the previous owner tried messing with it. That's a problem because that's set at Rochester. If the throttle plate closes completely, this is a good indication of the screw being messed with.

Does it stall at high temperature and when it's cold?

petermegan
06-12-2012, 12:10 AM
Bloody hell, that is one hell of a run down on your motor.:worthy: