I started a thread on OSO regarding the engine breaking up last fall. BUP - had some good info to offer and thought I would see what your thoughts are for the following:
Now when I start the cold motor, I can turn the key and it fires but breaks up real bad. I can turn the key back off and immediately back on and it fires off and sounds normal now. Sometimes I do need to throttle it up and clear out the fuel and then it idles fine and runs fine. I did a fuel pressure check and it was right at 40. There is definitely a problem with it sending too much fuel (or not being able to burn off the fuel...spark) for it to burn off right away. I can tell it is the #7 cylinder and by looking at the exhaust flaps that side is not quite right. So I checked the cost of fuel pressure regulators and they are quite pricey to just replace. If I am correct, there is one at the cool fuel on the side of the motor as well as one at the fuel rail. Do you have any ideas?

For my 383 build I bought some adjustable ones from a Volkwagen Golf. If you want to go that route let me know and I can look them up.

I don't know the history but I'm guessing you've done a lot of basic things. Have you tested all your injectors?

I recently fixed a stock low mileage (believe it or not) 92 Mustang 5,0 that was doing the exact same. Drove me nutz. Ended up being the main EFI relay. Fukr. LOL.

Sorry it wasn't a Merc motor but figured I'd throw it out there.

Thanks for the reply.
Yes did a few things: changed the one injector for #7 cylinder, all spark plugs, all wires, coil pack for #7, fuel pressure check 40psi. Did not do all injectors simply because we felt was #7 issue. Just feel since last weekend there is a lot of fuel being dumped or not burned off completely. Since we changed everything spark related think it is now fuel pressure regulator or pump.

The relay would click on/off many times a second causing the ****ty running. Turn key off (de energize relay) turn back on (which re-energizes it) and it would be fine. I finally gut lucky when my back was hurting, so I was kneeling on ground with head in pass side firing up my scanner and i heard the faint click click click click click behind/next to glove box. Why I was on this side I don't know. Scanner connector driver side rear engine compartment. Doh !

I believe 40 psi is correct spec.

If you think the reg is leaking fuel, pull the vacuum line and see if there is fuel in it. I'd check for this leak when key turned to run, not with engine running in case fuel does come out of it.

Jeremy,
Definitely double check the fuel pressure specs. I thought it is supposed to be 43 pounds.
There are two fuel pumps for each motor, a low pressure and a high pressure. I don't remember the specifics, but I definitely had to change a couple of fuel pumps over the years on my 496HOs.

42 to 43 psi - for the high pressure side -- 10 / 11 psi on the low pressure side

Have to run to go work on guess what a 496 - Have 3 in this week. Had to throw that in since we are talking 496 here. --- 2 merc and one volvo

Intake manifold gaskets - if its oil not fuel - how is your oil levels ???? and transom soot ?