Mystery "Check Engine" light

Hi all. I have a '90 5.0 LX Mustang (stock EFI), with 5-speed manual.

I recently had a complete engine rebuild, based, in part, on checking the engine after seeing a steady 'check engine' light. It turned out to be a spun bearing.

So, now I have a completely rebuilt (basically stock except for a .030 overbore, hyper pistons, ARP hardware, and ported heads) engine, plus new stuff like water pump, oil pump, fuel pump, fuel filter, clutch, valve springs, HD timing chain set, etc. Basically, I replaced everything I could think of while the block was out. While the engine was out, I found that one header was cracked, so I replaced the mystery headers I had with a new pair of ceramic Flowmaster equal-length shortys, which now mate to my 2.5" Magnaflow H-pipe exhaust.

And, to keep a better eye on the condition of my engine, I installed a set of mechanical gauges (oil pressure, oil temp, coolant temp). Everything is working well withing the correct operating pressures/temps, and the engine is strong and sounds great.

Anyway, I'm still getting an intermittent 'check engine' light, mostly during medium to heavy acceleration, or while increasing load (climbing a steep hill, for example).

I'm thinking that perhaps the 'check engine' light is being triggered by a false reading from an oxy sensor. Is that possible? My Magnaflow exhaust has only two cats total, as opposed to the four cats with the stock exhaust. Could that be a factor?

I KNOW there's nothing wrong with that plant, so what the heck could it be??
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maybee it's from bypassing your stock gauges (removed/unplugged relays).. Have you had a diagnostic test? The computer would tell you quickly. It cost $50 in my area or you could by a compouter code reader in the parts store.

someone got better ideas???

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LET'S GOOOOOooooo

<font size=-1>[ This Message was edited by: tbirder on 12/28/02 4:21am ]</font>

I left all the stock gauges/sensors installed and hooked up, just in case. The additional gauges are "T"-jointed to the original fittings.

I guess I'll do a diagnostic. I was hoping it was something obvious I'd missed. If the diagnostic doesn't show anything, and the air/fuel mixture is where it should be, I'm just disconnecting the damn light. [img]/phpBB/images/smiles/icon_rolleyes.gif[/img]
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Many people can't find the reason for the light. When I did my swap, I knew the light would come on... I pulled the bulb... problem fixed[img]/phpBB/images/smiles/icon_smile.gif[/img]
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Hey dude! I have the same problem on my 1992 5.0 LX. Ofcourse my problem is more complex with all the mods I have made. Still can't figure out why the light is on. The codes came up oxygen sensors. Spent $70.00 each and did not fix problem. So you can't trust that theory either. Hope you find a fix. Check out my ride at (billyrickles.com)Pictures have not been upgraded since the window tint, engine dress up kit, and new wheels &amp; tires.

Thanks! I'll check out your site.

By coincidence, I happened to remove the MAS today to get access to put in new plug looms, and when everything was back together, guess what? No 'check' light. So now I figure it's just a short somewhere. I think I'll just pull the bulb.

Ultimately, I'd like to get it sport-tuned and get rid of all the emissions stuff, since I live in an area with no annual inspections. Then it'll be nice and simple to keep an eye on everything.
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Dirty sensor wires in the Mass Air meter will cause oxygen sensor codes, and the interminent check engine light. I also changed o2 sensors for this. After the were replaced still got o2 codes plus the air meter code. Cleaning the meter fixed it, been over a year and no problem.
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That damn light is back!! I'm going for a diagnostic on Monday. It's with the place that rebuilt the bottom end, so it's "on the house". I'm curious as to what they'll find...
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I have a 93 LX and I've had the same problem for about 4 years now. More prevalent in the winter months. Just as the car is reaching normal operating temperature, and the engine has a bit of a load on it, at highway speeds, the light comes on for a second or two. This continues for about 5 minutes and just stops. I'm curious if you solved your problem, but it sounds like I better clean those wires and see what happens.
Tony Lopez
East Rutherford NJ

"I'm curious if you solved your problem..."

Sure did! I just pulled the light, and here's why:

I took the car to they guys who rebuilt the bottom end and heads. They have warrantied their work for a year, so they have a vested interest in making sure the motor is running they way it should. They did a diagnostic, which revealed a loose/broken vacuum hose. Fixed the hose. Still, "check engine" light. They spent another three hours poking, prodding, and even dyno-ing the car, and still couldn't find why. They completely cleaned the MAS unit and traded the oxy sensors with a '90 GT they had in the shop. Still, the light did its thing.

So, it's safe to say that the light has nothing to do with the performance or condition of the powertrain. It's probably just getting unusual signals due to a lack of cats on the aftermarket exhaust. Absolutely everything else is running 100% tip-top.

So, since I have a full set of gauges to keep my eye on the motor, I just opted to pull the bulb, with the machine shop's blessing. Kind of a Homer Simpson approach to car repair, but so far, so good!

One note: the machine shop guy is very helpful, but he's an old-time Ford guy, so, after an hour of deliberating, he stated plainly, "Well, I think I found your problem. Looks like some idjit replaced your carb with fuel injectors!". As a comedian, he's a great machinist [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] !
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