72T running rich, possible mfi issue

[Boosted3s]

Hi guys,

I have finally gotten my 5 year 72T nut and bolt restoration to where I can drive it and I am running into an issue. The car will start right up when cold and drives perfectly until I shut it off, once I restart the car hot it acts like it is so rich it backfires in the exhaust and is barely driveable. I have had this happen both times I have taken the car for 15 minute trips, shut the car off and restarted a few minutes later, it goes from running great to undriveable, I have pulled the plugs both times and they appear very rich, all six plugs look the same too. Here is a bit of history on the car. The engine was restored and has good compression, the mfi pump and injectors were gone through by mfi werks. Throttle bodies were restored and I spent a long time getting the linkages all set up. The distributor was rebuilt and ran on a distributor dyno to confirm the timing curve and dwell. Ignition timing was correct after engine was installed, it’s only been driven 20 miles. The preheat hose from the heat exchanger to the mfi pump is there and in good shape. Spark plugs are NGK Bpr7es. I am not sure what to look at to try and correct this issue.

[RSTarga]

Are you getting the car up to temp? 20m is not very much. Take it for a longer drive before shutting down.
Nice car

[Boosted3s]

Thanks. Both times it has been at about 180 before shutting it off, I will try a longer drive tomorrow.

[NorthernThrux]

If it is set up right it should make zero difference whether you drive 5 or 50 minutes before you shut it off. I will often back my car off the lift and leave it rinning or not for the 10 minutes it takes me to get my Macan onto the lift and up before leaving and it makes no difference how long it was off for.

The enrichment sources are the cold start solenoid, the mfi thermostat and the flap in the flapper box (indirectly). The flap doesn’t do a whole lot other than control the air intake temperature on a cold start. So let’s discount that. It’s possible your cold start valve opens on restart and dumps fuel into the intake. You don’t say how long this lasts and if it sorts itself out after a few minutes. That would be symptomatic of a bad thermo-time switch and maybe a sticking cold start valve. Disconnect the cold start valve electrical and see what happens. Also pull off the braided hose at the solenoid and see if it is leaking. For the MFI to run rich like that is odd as the control rack would have to go all the way back to its rich position. Possible if the thermstat discs are not setup right, but odd if MFIWorks went through it.

When you are restarting hot, are you depressing the accelereator all the way when you crank it? That might be causing something in the mfi pump to get hung up. Try start with only the hand throttle and see if that helps, even if it is harder to start initially.

Beautiful colour BTW. An your engine bay looks clean enough to eat off of.

[edmayo]

After your drive put your hand on the thermostat housing, it should be to hot to keep your hand on it.

[Boosted3s]

Thanks for the suggestion of things to check. I drove the car farther today and the same problem slowly came on as I drove, it seemed to start when the temp got to about 185 and progressively got worse. The thermostat was definitely hot to the touch when I got home. One other thing, I disabled the cold start by plugging the fuel line from the solenoid to the jets. The thought of relying on 40 year old plastic parts to hold in fuel scared me a bit. I am thinking about installing a wideband O2 sensor to monitor afr’s.

[Richy]

I predict it may be too lean . The spark plug does not look rich for an MFI engine .

[MattB]

Thread the end of the thermostat housing cap so you can externally adjust the enrichment? Quick and easy to play with it to see if you're too rich or too lean, and if the system is affecting things.

[taldora_2008]

As Richy (above) says, your MFI will also backfire if it's running to lean. You are "flying blind" unless you can measure the AFR. I'd also check that there are no exhaust gasket leaks as any air sucked in will cause backfiring. Nice looking rebuild, by the way.

[NorthernThrux]

If it gets worse just driving as things warm up, then my guess is you are getting too lean, but are OK at cold start. Sounds like it gets worse even in the absence of shutting down and restarting?

Remove the hot air hose to the MFI thermostat and see what happens. That should remove the leaning effect of the thermostat and give you some idea of what is going on.