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Author Topic: Stalling engine and contradictions **SOLVED**  (Read 725 times)
johnmcbridehouston
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« on: March 02, 2020, 12:42:19 am »

Somehow I messed up this posting, it is a continuation of my earlier posting with the same title.

Well it is the usual one step forward and two back.  After further  checking and testing I decided I was not going to get any further forward without improving my diagnostic abilities so I bought a TechMate Pro code reader and test unit. The first thing I found out was that there were no current codes to read, there were a couple of old ones a low voltage and an overheat (engine does not like the hose and a fast idle) but nothing active. I had already replaced the electronic boost control unit so moved onto the Electronic throttle control with the Techmate you can open and close this in five degree increments while monitoring the read backs, the flap did not move and the read backs did not change on 0 to 100% though there was a “fluttering on the valve as if it were trying to move but could not. I thought I had solved the problem and ordered a used but good spare from eBay to test it. The test unit arrived and I plugged it in to test it. One thing I did notice is that when the test starts the flap moves about three to five degrees but nothing after that it also had the same fluttering as the original unit. There is power on the unit as it will not move while under test but once taken out of test the flap is movable fairly easily and both read backs change as it moves. I went back and looked through the other readings and there is something really odd happening, the TechMate is showing a boost pressure of 94 psi (the supercharger can only put out ~28psi and that is absolute pressure) not only that but the engine is not running. Manifold and barometric pressure both reading 14.8 psi TPS volts (ETC position feedback) was reading 0.86 volts which is its closed position.
I think that what is happening is that the PCM is reading this boost pressure and closing the ETC to try and reduce the pressure, this is further backed up by  the boost control voltage being at 94%  (the ETC has both closed off the air intake and opened the bypass to reduce pressure) the  TechMate test  is trying to override this and open the ETC and the constant open/close signals are what I feel as the “fluttering” what I do not know is where the boost pressure signal is coming from, according to the manual the MAP sensor is the only air pressure sensor on the engine and this is reading correctly Manifold and barometric pressure both reading 14.8 psi which is what they should be with the engine stopped. I will try to upload the pictures showing the various readings, but does anyone know where the ETC is getting this signal from?


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« Last Edit: March 14, 2020, 08:51:43 pm by johnmcbridehouston » Logged

Trask
2006 250 HP Verado 1B459812 ~500hrs
johnmcbridehouston
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« Reply #1 on: March 09, 2020, 10:09:14 am »

Hi Guy's

Does anyone happen to know  what the start/idle TPS voltages should be? having a code reader is fine and gives a lot of information about actual readings but not so much about what they should be. I am showing ~0.85 at start the ETC looks pretty much closed but I still get the occasional "engine getting too much air" when she stalls and the manifold pressure is around 7psi, I seem to remember seeing a couple of screenshots showing around 3 (boost at idle). When I get this running again I will record and post all idle data shown by the scanner.

Thanks
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2006 250 HP Verado 1B459812 ~500hrs
johnmcbridehouston
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Posts: 20


« Reply #2 on: March 14, 2020, 08:50:12 pm »

Hi Guys

I had got to the point where I had proved to myself that there was nothing wrong with the engine (except that it would not run…)
Gas replaced, all filters renewed, gas drained and replaced, batteries tested, new plugs, new coils, no air leaks, fuel pressure 52 psi, MAP sensor good showing good reading for barometric and manifold pressure, throttle valve good, Electronic boost valve good, all connections parted and cleaned. No codes showing nor stored, plugs all the same color so not a plug or injector problem. I was sitting at the helm wondering how I could drop the motor off the back of the boat and looking at the engine screen of the GPS and looking at the fuel pressure when it hit me. Fuel pressure was reading 52psi, it always read 52psi! but fuel pressure should be :-
Manifold/boost - Barometric = X + 50.76 = fuel pressure, the point being that fuel pressure should change as the manifold pressure changes the engine page did not show this. Turns out that there is no fuel pressure sensor on a Verado engine. If you are reading it on any system it is a calculated pressure. I bought a fuel pressure test set and put it on the fuel injection manifold and sure enough the REAL fuel pressure was ~20psi just enough to start and run the engine even if very roughly and with frequent stalls. Nowhere in any forum have I read anything that results in a low high pressure. I bought new high- and low-pressure pumps, regulator, fuel level float switch and repair kit. Rebuilding the FSM is a fairly easy task and with the new parts the fuel rail pressure (REAL) varies between 48 and 52 psi and the engine is running fine between 500 and 1500 RPM though I still have to take it out for a WOT run I am confident that the problem is solved.
 I still do not know why the throttle valve would not test but I do know that it is working nor do I know  where the 94 psi boost pressure came from, when I asked Mercury customer support they told me that they do not monitor boost pressure?? if I ever find those answers I will post them here but don't hold your breath.

John
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2006 250 HP Verado 1B459812 ~500hrs
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