5.7L TBI Mercruiser, wont idle

[msmcada]

Hello, working on my 2000 Rinker with a 5.7 TBI Mercruiser I bought last year. Boat looks like it has been sitting for a long while with bad fuel and the whole nine yards, working out all the issues. Fuel was removed and replaced to start with all filters and seperators replcaced and lines flushed with fresh fuel. What it does now is starts, idles perfect for about 6-10 seconds, then dies. If I bring up the RPM's to about 1500 or so, it will continue to run, then try to die and I have to feather the throttle to keep it going, then it will try to die again. So I unhooked the oil pressure senders, still same issue. Unplugged MAP sensor, same issue. Unplugged the sensor at the back of the throttle body, same thing. Unplugged all sensors on throttle body (idle air control IAC, throttle postioning sensor TPS and the one on the back of Throttle Body, not sure what it is) and the engine idles like a dream. So started pluging sensor back in, the result was if both the IAC and TPS are connected the boat will not idle. So thinking the IAC and TPS are bad I ordered and replaced both items and guess what, same thing engine will not idle.

Any ideas would be greatful...

Thanks

Mike

 

[boat127]

Re: 5.7L TBI Mercruiser, wont idle

the sensor on the back of the throttle body. is it brass? if it is and goes into around where the fuel line is located it is a fuel pressure sensor. I would remove the IAC and clean it out it maybe carboned up. what you can do also is when you run the engine is hook up a timing light and remove the flame arrestor but don't lean directly over it and aim your timing light at the injectors and should be able to see a good spray.

i would also remove the distributor cap and have a look at the cap to see if theres any corrosion. also have alot at the ignition sensor in the distributor and see if it's corrored they are known for problems and can cause a surge. does this have the cool fuel pump thats mounted on the port side of the engione down in the bilge by the engine mount?

 

[msmcada]

Re: 5.7L TBI Mercruiser, wont idle

Thanks for the reply. The sensor on back on throttle body is brass. I
have replaced the IAC, MAP and TPS on the TB. The injector spray looks
like it has pulse and a nice pattern. I have also replace all the
distributor components cap, rotor bug, sensor pick up in bottom of
distributor and wires with plugs. This engine does have the fuel
cooler on port bottom of engine...

Mike

 

[Don S]

Re: 5.7L TBI Mercruiser, wont idle

Have you done any fuel pressure tests? Low and High pressure?
How about a compression test? Did you replace or even look at the spark plugs to see how they were burning?

 

[boat127]

Re: 5.7L TBI Mercruiser, wont idle

have you clecked for vacuum leaks around the throttle body and intake manifold with some brake cleaner? does it back fire at all? i would also talke alook at the ani siphon vlave on the gas tank to see if its partially clogged. another thing you can do is hook up your timing light and see if looses sparks when it starts to surge. also hook a multimeter to the coil and see if you have 12 volts when its running and see if it drops when the surge happens.

 

[msmcada]

Re: 5.7L TBI Mercruiser, wont idle

I have not done fuel pressure test. The old and new plugs look great visually. The engine will idle fine without the TPS hooked up or mounted to the throttle body. New discovery today, if you unmount the TPS and leave it connect to plug the engine will idle but as soon as you mount on throttle body the engine will no longer idle.

Mike

 

[msmcada]

Re: 5.7L TBI Mercruiser, wont idle

Yes I have sparyed carb cleaner around TB and intake and not change in engine RPM. During the trouble shooting process I have intruduced a new smaller tank with fresh fuel only to have the same result. New discovery today, if you unmount the TPS for throttle body and leave it connect to plug the engine will idle but as soon as you mount on throttle body the engine will no longer idle. Wondering if there is some type of calibration in the ECM for TPS? or maybe something incorrect about ECM?

Mike

 

[Don S]

Re: 5.7L TBI Mercruiser, wont idle

You need to get hooked up to a diagnostic computer and find out what is going on, instead of just throwing parts at it.
You could have bought one for the price of the good parts you replaced.

http://www.rinda.com/marine/mtspec.htm#techmate

 

[msmcada]

Re: 5.7L TBI Mercruiser, wont idle

Are they very expensive? I was told that they do not diagnose much...

Thanks
Mike

 

[Tail_Gunner]

Re: 5.7L TBI Mercruiser, wont idle

Fuel pressure gauges can be rented from carquest....If your iac wasnt working you should get a high idle speed say 1500. The fact it runs unplugged well should be the issue. Its just a gm tb go to a auto house and get a new one...ask them if they take trade ins....

 

[msmcada]

Re: 5.7L TBI Mercruiser, wont idle

I think the issue has been found... I bought this boat used and not sure if it ran, had looks like it had been sitting for a long while. After spending time on the phone yesterday with Arizona Speed, looks like the ECM has the wrong program. It has the E930 program in it and that program is for a Multiport 5.7L and not my TBI 5.7L. So sending ECM off to be reprogrammed today.

Wish me luck!

Mikw

 

[Don S]

Re: 5.7L TBI Mercruiser, wont idle

Good luck, but I find it hard to believe that you could even get a TBI engine to start with an ECM designed for an MPI.

 

[Tail_Gunner]

Re: 5.7L TBI Mercruiser, wont idle

Good luck, but I find it hard to believe that you could even get a TBI engine to start with an ECM designed for an MPI.

makes no sense at all. Mpi requires batch firing...aka your firng 8 injectors with mpi and tbi runs two. I cannot see two injectors pulsing at that rate without serious flooding going on..actualy they would be in a static mode that is under constant fire or continous operation.. and it runs smooth with out the tps in the circut...?

 

[Dshow]

Re: 5.7L TBI Mercruiser, wont idle

I find it hard to believe that your ECM has the wrong software, it would have never run in the first place. Has the ECM been updated or replaced lately? Where you reading back a checksum from the ECM to determine it was the wrong software and if so, what else did the scan tool say?

 

[Tail_Gunner]

Re: 5.7L TBI Mercruiser, wont idle

Not the wrong software..it would not even start is it was. Its more than probably a mefi-3 system, what he is saying the softwre was programed to run a mpi injection system...while it would time the motor and run the engine equipment properly it would signal the injection system to fire 8 injectors at certain pulse times and duration...Now the TB he has has only two......Thats not going to play out well...or be at bit confusing

 

[jeremyhuston]

Re: 5.7L TBI Mercruiser, wont idle

Actually.. The difference between Say TBI PCM/ECM and MPI/CPI is simply a "constant" in the ECM/PCM. It would start.. and it would run like crap. The difference is the amount of sprays per revolution. A TBI system sprays either once or twice and the others are the opposite. (memory is faded).. Totally blown away someone would get the wrong ecm? guessing there was prior issues that someone tried to fix with the wrong diagnosis? A simple way to check your TPS sensor is with a Multimeter. .5 at closed throttle and WOT should read 4.5v im memory serves me correct.