Backfire On Startup And Throttle Down


New Member
Mar 24, 2014
Hell, I have a 91 Mustang 5.o 5 speed convertible. I am very very new to the site so please bare with me if im lost. Ill start out with me, broke engineering student with my first mustang.

Bought my stang about 6-8 months ago and have put about 7k on it, while ago it starts poopin white smoke and after checking a few things I come to the conclusion that its a blown head gasket and being fairly broke, I decide to do a top end rebuild myself. Hell of a time with a few things but everything is back on the car and it runs with no leaks that I am as of yet aware of (timing was set to about twelve, bumped up from ten). Now for the issue at first start up it read out a few blink codes one for a low voltage on the egr, one because I had forgotten to plug the MAF in my excitement, plugged the MAF in and pulled the egr and cleaned it, holds vacuum and the switch is nice and springy, that cleared both codes. Now everything is back on the car and I have no codes running, or koeo BUT I have low power and im running real rich I think and this is with new plugs (Bosch platinum) and wires. Not only that but it now idles low (round 650 as opposed to the normal 800ish) and rough, backfires on startup, and backfires most of the time when I let off the throttle. This gets worse when it's under load. Currently has a little "old" gas (like a quarter tank of two month old gas) mixed with 91 octane and at my fathers suggestion a bottle of octane booster. Engine is entirely stock but with somebody elses offroad H pipe. I have tried dialing the advance back, went so far as 8 but it seems to idle better at twelve than 8 or 10, suggestions?
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Founding Member
Apr 27, 2001
Cincinnati Ohio
Does it idle rough? I'd start by checking your firing order to make sure its correct (assuming you had the wires removed) and make sure they are all (the plug wires) are all pushed on all the way.