502 Mag MPI (VST) surges and stalls
 

I have run into an engine that cold starts great, warms up in idle and when getting close to normal temp it starts to surge, first barely noticable but it gets worse and worse until it suddenly stalls.
Scan tool first led me to a faulty air temp sensor, showing a steady 100 deg F on a 60 deg evening. A new sensor was installed tonight and it started out showing around 60 as expected. After a couple of trials the engine again starts to surge and stall. Once again the IAT sensor gives around 100 deg F but by that time I was to exhausted to notice.
Fuel pressure is ok at idle, it remains high when shut off (no leaking injectors), TPS is good, IAC has been reset and seems to be working, coolant temp sensor is ok, MAP sensor is ok. No fuel is leaking from mech. pump or VST into the plenum.
Fact: the old IAT sensor smelled gasoline ...
My thoughts: could it be a leaking intake valve, allowing compression to blow fuel back into the plenum, messing up the vacum signal to the ecm and causing an over rich condition? When stalled the engine starts a lot easier in "clear flooding mode", i.e over 50% throttle.
Anyone?

Your VST says it all. There is a micro-screen filter in the VST unit. Most likely you will need to change it (expensive for the itty bitty oart).

You have most likely gotten some dirty fuel somewhere. Change the external fuel filter, work on the VST and you should be good. Also search for VST in the forums. There are several god threads that cover this subject. Hope you get it running right.

It sure sounds like you have checked all the right things, but something is causing the engine to load up and surge. How do the plugs look? Don't be shy about replacing them if they are blackened. You may have to go through several sets, but you have keep clean plugs in it or they will make it that much harder to troubleshoot. Have you checked compression?

I don't think the IAT is causing your problem, if it was showing a higher than actual temp, it would be leaning the engine out, not making it richer.

Dave

What I wrote was how far I got last night. Next will be a compression (and plug) check and a intake vacum check to try to find signs of leaky valves. The IAT is probably not the source of the fault, what struck me on the way home was that the new one ended up just as the old one, i.e nailed at 100 deg F after the engine had been running for a while.
I bet (but not yet confirmed) that the new IAT also got soaked with hot fuel which is what is leading me towards the valves.

Fuel in this case must be spitting up from one of the intake runners since the injectors are sitting app. 4" down from the plenum. Neither the VST or the mechanical fule pump is leaking fuel into the plenum so ... ?

I'm sorry but I can't see what the screen filter in the VST would do in this case besides probably leaning out the mixture.
This would for sure have killed the engine during my test disconnecting the IAC valve. What actually happened was that it ran on high idle (~1300 rpm) and by that most likely managed to burn the excessive fuel ...

If the compression check doesn't lead me anywhere I'm gonna start borrowing parts from the port engine, that includes the VST

Well, todays exercise proved my theory to be correct. A compression test showed zero pressure on cyl # 4.
Sad news on an engine that's supposed to be rebuilt not too long ago ...

my 01 502 wont run under 800rpm stalls out but runs great anyting above 1000 need help

Well, without any basic troubleshooting advice is very hard to give. As you may see from my posts I have checked the engine sensors, fuel pressure, manifold vacum and finally compression. Tools I used were scan tool, fuel pressure tester, compression tester, vacum tester.
Of course knowledge and understanding of the engine management system is mandatory, otherwise you're just aiming blindfolded.
What info can you provide?

Sad news indeed Alex. At least you were able to figure it out.

Bring
Out
Another
Thousand

Hey Foolinjected, You might get more responses to your problem if you started your own thread.
:D