454 mag mpi chuggles/stalls

[vroom ZOOM]

Ok, this thing has me beat. 2000 454 mag mpi (mefi3). starts, then when settling to idle it chuggles and stalls. Brand new longblock. old one also did the same thing. I tried known good ignition modules from properly running engines, reset the distributor to #1 tdc countless times, tried a different wire harness, ECM, brand new OEM injectors, tested and verified to spec at a reputable injector shop. I took a complete intake with tps, iac, and IAT from a running engine and no change. Also a new alternator, pickup coil, and ignition coil. and I spent 2 days trying every position possible on the distributor and no change. When the engine starts the vacuum jumps to about 20 and then settles to about 16 and then it starts to chuggle, the iac opens, the vaccum drops and the engine stalls. Fuel pressure is at 43 psi. Running it off a remote tank with fresh fuel. No codes. 5.19 volts on the 5v reference circuit.
I can keep it running if I pump the throttle. Also today morning I unplugged the map sensor and it started and stayed running, albeit a little too fast. Then a little while later I tried starting it with the map sensor plugged in and it also started and idles but too slow and a little lopey. I thought it might not have enough timing so I advanced the distributor but that did not help. I set the dizzy back and tried running with the map sensor disconnected but now no change. I think I tried everything here I could use some help.

EDIT: I forgot to mention that I tried throwing in the cheap chinses knockoff injectors from the running engine. That made the engine run excellent. I took them to the injector shop and was told that the knockoffs had the same flow rates as the OEM injectors, but with a much larger flow variance which should theoretically make the engine idle worse... also they told me the impedance was the same as OEM.

 

[alldodge]

Have you tried to plug the PCV valve?

Use a NOID light to see if all injectors are firing

Might have a bad ground connection in the harness

Could use some real time voltage readings of IAC, MAP, TPS and IAT Sensors

 

[vroom ZOOM]

Have you tried to plug the PCV valve?

Use a NOID light to see if all injectors are firing

Might have a bad ground connection in the harness

Could use some real time voltage readings of IAC, MAP, TPS and IAT Sensors

Yes I plugged the pcv valve when I put the vac gauge on it. No change. Will try it again and post screenshots of diacom. I tried the harness from the working engine, no change.

 

[vroom ZOOM]

Have you tried to plug the PCV valve?

Use a NOID light to see if all injectors are firing

Might have a bad ground connection in the harness

Could use some real time voltage readings of IAC, MAP, TPS and IAT Sensors

ok, I ran the engine a couple of times and generated some graphs.

 

[alldodge]

MAP sensor shows good readings

IAC position list 40 (where it should be) but graph shows it's wide open trying to give it more air. As IAC opens the pulse width gets less which is opposite of what it should

Pulse width shows it's try to give it less and less fuel

Unplugging the MAP causes more fuel to be supplied

Hay @muc have any ideas?

 

[vroom ZOOM]

MAP sensor shows good readings

IAC position list 40 (where it should be) but graph shows it's wide open trying to give it more air. As IAC opens the pulse width gets less which is opposite of what it should

Pulse width shows it's try to give it less and less fuel

Unplugging the MAP causes more fuel to be supplied

Hay @muc have any ideas?

Well here is a REAL head scratcher:
I put the cheap knockoff set of injectors in it that the other running engine had last week and that made the engine run PERFECTLY. I thought that the injectors were bad so I ordered a new OEM set. I put them in and it runs like crap. A few hours ago I decided to throw the cheap knockoff set in and guess what? The engine runs PERFECTLY, without a hint of stalling. I could probably balance a coin on it with it running.

I put the noid light on the OEM injectors and when the engine stalls the light is still pulsing on both the green and blue injector circuits.

Since the engine runs amazing with the cheap knockoff injectors, I think the following is also working properly:
Distributor/module/pickup
Timing is set correctly
All the sensors are working properly
No vac leak

I feel like there is an elephant in the room that I am not seeing. Something incredibly DUMB.
Here is a video of it running with the fake injectors:

 

[alldodge]

Strange
Does the IAC still show it close to 100?

 

[tpenfield]

Got a graph with the TPS reading during this recording?

 

[vroom ZOOM]

Got a graph with the TPS reading during this recording?

Ill crank it up today and post the graph.

 

[vroom ZOOM]

Strange
Does the IAC still show it close to 100?

no, iac is around 50 counts.

 

[tpenfield]

Ill crank it up today and post the graph.

Thanks . . . thinking it may be helpful. Do you have the recording file from before? Could just use that to make the TPS graph.

 

[vroom ZOOM]

Thanks . . . thinking it may be helpful. Do you have the recording file from before? Could just use that to make the TPS graph.

no unfortunately I dont. I really didn't record anything as the engine ran amazing. Will do that today.

 

[dubs283]

Check the vacuum lines that connect to the fuel damper/regulator at the back of the fuel rail, port side of engine. There should be no fuel present in the line

That damper can fail and cause an overfueling/rich condition at idle. It will be evident by checking for fouled spark plugs

If this is indeed your cause, the damper is nla from merc and GM. In the past I've plugged the damper vacuum connection and the vacuum line at the damper to solve the issue

 

[vroom ZOOM]

Well everyone, it is FIXED!!! FINALLY! you are not going to believe what it was...
The fake injectors had the pins in the connectors slightly off center, and they made better contact with the terminals in the socket that had taken a "set" and had a loose connection. So the fake injectors made better contact and ran. No corrosion on the sockets anywhere. I slightly bent the pins on the oem injectors and now the motor runs perfect. It was probably a slight bend by the amount of the thickness of paperclip. Just that tiny difference not visible to the naked eye did the trick. Needless to say it took me a long time to figure out.

 

[vroom ZOOM]

Well DAMN! I spoke too soon. As soon as the boat hit the water, the motor is back to its old self. I checked the contacts on the injectors and now that doesn't help. I swear this thing is cursed. Restart the suggestions!

 

[alldodge]

It doesn't run, then it does, they it doesn't
Bad connection somewhere, and that as you know will be hard to find

 

[vroom ZOOM]

It doesn't run, then it does, they it doesn't
Bad connection somewhere, and that as you know will be hard to find

oh jeez....

Just spent the entire day in the engine room testing every possible circuit on the wire harness, and no luck. I even took some pins out of the ECM plug and did static tests on the injectors, each one pulled 0.75 amps through the wire harness and directly, indicating a good connection. Keep the ideas coming!

 

[cyclops222]

Have you checked ALL / each sparkplug to see if some are different ? Like the start of a head gasket starting to leak ? Been there and hated the random problem.

 

[tpenfield]

OK, since you have the Diacom software, my suggestion is to connect it when the engine is running (i.e. cooperating) record a few minutes of data, Then do the same when the engine is not cooperating. (you won't get much data, but run it while you are cranking the motor, etc.)

I would expect that there is something intermittent (as mentioned) and the recording may provide some insight. PM me if you want to send me your recordings.

 

[tpenfield]

BTW - I had to Google "chuggles" . . .

"Chuggle" can refer to two main things: a type of dog breed or the act of drinking something quickly. As a dog breed, a "chuggle" is a cross between a Chihuahua and a Puggle (itself a Pug and Beagle mix)

Perhaps your engine is 'drinking too quickly' . . .