2000 GT 4.6 Convertible Puzzler

TheDHndrsn

New Member
Sep 12, 2011
10
0
2
Casa Grande, AZ
Dont post here much. Love my GT convertible but rarely o much to it. Recently had to do a small bit of maintenance. Now faced with something I cannot wrap my head around. Here is what we have/

GT 4.6 154K miles
Maintenance
replace of Upper Intake Manifold and Gaskets
new alternator
New Valve Cover Gaskets
Injector Maintenance
New Battery
Oil Change and Filter (full synth, high mileage)
Coolant Flush and Clean

Started it up and it purred like a kitten. Put it to bed for the night.

Next day it fired up perfectly. Backed out of garage and it began running rough. Did not even get it back into garage. Would NOT restart.

Neighbor is a professional mechanic. He brought by his super-duper, worth as much as the car tester/code reader after work. He spent more than an hour analyzing. Nothing. No codes. No failures. Every test he ran passed with flying colors. The car spent the night outside. Attempted restarts failed.

Next afternoon while waiting for my neighbor again we tried starting. First start attempt since previous day. Fired up and purred like a kitten. We pulled it into the garage.

Allowed it to idle for a minute or two. Smooth and effortless. Decided to rev to about 3K. Within 15-20 seconds it began running rough, Backed off throttle and saw a smooth idle 500-800 rpm. 30 seconds later it simply quit. As if someone had switched off the key. Would not restart.

Neighbor arrived. Repeat of previous day. No issues identified. No codes other than an occasional miss related to a single injector.

24 hours later it started right up. Tested fuel pressure at rail. Just under 40psi and solid before, during run, and after it died. Same scenario. Starts. Runs smoothly. Dies after it starts to warm or rev. Zero codes or errors to report.

Continuity observed on everything we could think to test. All leads / connection identifiable solid. Even pulled air filter from end of intake in case that was restricting flow.. Nada. Nothing.

After the engine dies attempted restarts give gas smell. flooding. No spark apparently although it is not throwing a code.

What could lead to engine shut down while not throwing any code but only after a slight warm up or period of time? Security system? A troll under the hood?

Any ideas of what to look at with this? We need some fresh thinking. Lost here.
- Dave
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,894
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Houston Texas
Just wondering. HOW did you measure fuel pressure? With an external fuel pressure gauge? Why is this important? Because if measured with an external fuel pressure gauge 40 PSI is too high for a motor at idle. Should be closer the 32 PSI.

What about the fuel rail pressure sensor (FRPS) intake vacuum reference line? Is it connected and leak free?

Has the PCM FRPS reading been crossed check to the external fuel gauge?

If you have access to a high end scanner then you have access to PCM operational PID's. How about monitoring:
  • MAF flow
  • Fuel rail pressure sensor
  • RPM's
  • System voltage
During cranking.

It seems that after a intake manifold replacement forgetting to reattach the FRPS intake vacuum reference line is a real possibility. Just like having a not a fully connected FRPS connector is a possibility.

As for "other" possible causes, how about a charging system problem?

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Or a loose ground around the radiator core support?

1996+ Crank with no start check list
https://www.allfordmustangs.com/forums/4-6l-tech/336452-1997-mustang-wont-ignite.html#2984838
 

TheDHndrsn

New Member
Sep 12, 2011
10
0
2
Casa Grande, AZ
Thank you for the response. Idles nice and smooth. Floods out once it warms up then will not restart.

Ran start system tests as related and all in spec. This is not, seemingly, electrical.

We monitored all above described and thing out of spec with the exception of the MAF which was a bit high. Double, triple, quadruple checked vacuum connections along with everything else. If something is bad it is internal and not showing up no matter how we view it.

Have decided to pull everything and rebuild from the UIM up. The only thing we've been able to theorize is that the deminimus presence of OEM gasket with the replacement UIM is losing seal once things begin warming. Going to try a new, more robust gasket.

Will advise.
Dave
 

TheDHndrsn

New Member
Sep 12, 2011
10
0
2
Casa Grande, AZ
$350 wasted on two upper intake manifolds, $100 on various gaskets, $150 on an alternator, $180 on a battery, $125 on new injectors, $98 on plugs, $80 on coil packs, $70 on hoses, $55 on wire harness and connectors, $500+ on various genius mechanics who "knew how to fix it", and a month later the best I get is an impressive 3 minutes of smooth idle once every 24 hours. I sold the bitch to some Mexican kid for $800 out of principle alone. I am done with Mustangs.
 

OneSick99GT

Active Member
Nov 20, 2018
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Dayton, Ohio
www.youtube.com
I would have figured this out for free! It was something easy I bet. Most of what you replaced wouldn't be causing this issue. That's why you need a good scanner or something that can read charts.