97 5.7 efi alpha burning head gasket

[97searay5.7efi]

Hello, I have a 97 5.7 efi throttle body. just bought it, Cylinders 3 and five were zero compression. Found the head gasket burned between cylinders, the head was etched between combustion chambers and so was the block. I installed a new long block and used original throttle body and distributor and electronics. Being aware that replacing the long block may be just repairing the symptom of the problem, I have been watching the engine closely. Yesterday however it started to work very poorly, popping through the tbi while trying to get back on plane. So I did a compression test and #3 IS DOWN TO 125PSI also the plugs are extremely lean. Does anybody have an idea why this engine is running so lean? This lean out condition is apparently what caused the last engine to burn up.

Thanks

 

[dr_bowtie]

Re: 97 5.7 efi alpha burning head gasket

fuel pressure is not in correct parameters or the fuel map is out to lunch....

check fuel pressure 1st...on MPI it should be 45PSI on a TBI injection system it should be 13PSI...

you can regulate the fuel curve a little by adding an adjustable regulator and playing with the fuel pressure...more pressure = more fuel...


it's either that or larger injectors...MPI has 8 while TBI has 2...

if you have a TBI make sure it is the original 5.7L one the 5.0L one has smaller injectors and same with the MPI...

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

This is the original throttle body and injectors. Injectors are not dirty. i am suspecting there is something wrong with the computer???

 

[Don S]

Re: 97 5.7 efi alpha burning head gasket

on a TBI injection system it should be 13PSI...

Not sure where you got that information from, but the OEM service manual states 28-32 psi, not 13.

97searay, if you go up to the adults only sticky at the top of theis forum, and download manual #17, it has detailed instructions on diagnosing your TBI system.

 

[jtybt]

Re: 97 5.7 efi alpha burning head gasket

Could have intake leak or crack on those intake ports. Inspect the intake manifold.

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

Intake is perfect. is there any way to trick the computer into rich mixture?

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

I just took the t-stat out to see if running the engine at a colder temp would richen it up. It worked great until the temp reached 140 degrees then it started missing while trying to put it up on plane. ?????

 

[Don S]

Re: 97 5.7 efi alpha burning head gasket

Instead of trying to half-azz it to work, or try to fool the computer, why don't you do some diagnostics, find the real problem, then fix it properly.

Did you download the manual?

 

[dr_bowtie]

Re: 97 5.7 efi alpha burning head gasket

agreed...any fix will only richen the mixture until WOT is hit then the fuel curve normal set will take over...

you need to find the cause of the issue...a faulty MAP switch or MAF can cause a lean issue...you can trick the MAP switch to run rich up until WOT...

you need to make sure the injectors are working fine and are of the correct #/hr rating...I would assume they are...

If it was running that lean then all cylinder would exhibit lean mis conditions...beings it is a center 2 cylinders I would think there is more an issue with those 2 cylinders...either stuck/dirty injectors or a vacuum leak...

you can however have reversion going on around those cylinders and cause the same issue but that is a little harder to diagnos and other issue can contribute...

as for fuel pressure by all means go with what the manual says...automotive is a little different than boating....

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

Thanks Don I did download the manual, all of the plugs look the same - lean. Is the map sensor the same as the chev truck i.e. 95 5.7 throttle body? i am trying to isolate a bad sensor. I read where someone made their own code reader with a diode does anyone have the recipe for that? The injectors are correct and working fine i am going to test the pressure however.



Thanks for all of the help guys!

 

[dr_bowtie]

Re: 97 5.7 efi alpha burning head gasket

yes the MAP sensor is the same for all GM products...

if you have an access terminal then you short pins A-B just like a car and read the SES light if you have one...

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

Hello, I checked the code and I am just getting 12, 12 , 12. I purchase a new map sensor but am hesitant to install it. Maybe I should check fuel pressure next?

 

[Bondo]

Re: 97 5.7 efi alpha burning head gasket

Ayuh,.... Have you Checked,+ changed the Fuel Filters,..??

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

I am only aware of the water separator / fuel filter on the engine. Is there another filter?

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

I should also mention my boat has the cool fuel system.

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

I removed the MAP sensor and found that the weathertight grommet was deystroyed. Could this cause a lean running condition?

 

[dr_bowtie]

Re: 97 5.7 efi alpha burning head gasket

yes....

 

[97searay5.7efi]

Re: 97 5.7 efi alpha burning head gasket

Just an update, I replaced the map sensor and took the boat out for a test. It is still running lean, stumbling while trying to get on plane once it gets warm. So ... unplugged the temp sensor and there was an immediate difference, you could tell that the engine became rich, there was no beep from the buzzer however even after 5 minutes. I have replaced the temp sensor but checked the one I removed and it seemed to be functioning correctly on the ohm meter. I haven't tried the boat it yet since installing the new temp sensor, (hurricane is here today) but I doubt that it was the problem seeing that the old one is functioning correctly. I am still stumped and the local mercruiser shops in this part of the world do not have any experience with efi.

 

[Don S]

Re: 97 5.7 efi alpha burning head gasket

You need one of these, Then you can see what is going on with your engine.

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

 

[rodbolt]

Re: 97 5.7 efi alpha burning head gasket

typically any sensor failure will set it pig rich, if you dont belive it just see what you did with the temp sensor.
with no feed back the ECU will see about -30 to -50 degrees.
can be seen with a lap top.
same with the map sensor,unplug it or the manifold sense line and it goes pig rich.
like DonS said, find the lean issue.
could be clogged injector filters,faulty fuel pump,clogged cool fuel filter bad or restricted ful lines or tank draw tube.
ECU,s are relativly stupid and will react to garbage in even though it will mean garbage out.
you system is a simple speed density EFI setup. no MAF sensor.
TPS gives throttle angle, ign control module gives RPM information,MAP gives engine load information based on manifold absolute pressure(inverse of engine vacum) knock sensor gives info on engine detonation and the ECU reacts accordingly.
coolent temp gives the ECU info on engine temp for fuel corrections at cold or hot starts.
IAC simply corrects air flow for idle speeds when the throottle plates are fully closed.
its a very simple system but requires a certain amount of system knowledge and test equipment to properly diagnose.
the one thing the system DOES NOT monitor is fuel rail pressure, not enough and it runs lean,to much and it runs rich. but each injector has a filter. the TBI filter kit can be purchased at about any parts store.