Check engine light

[87GrandMarq]

I have an 87 mgm and I know some of them got check engine lights. My car I believe has one. My question is, is it functional or does it only come on when its overheating? Attached is a pic of the lights. Its an older pic, I have a new unbroken bezel in its place lol.


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[sly]

That's the red "replace engine" light. It only comes on with no oil pressure (below 6 psi) or overheating. Check engine light would be amber/yellow.

[Tiggie]

IIRC the amber “check engine” is where your “check level” (oil level) light is. I think it’s mostly a 1989 thing.

[87GrandMarq]

Oh ok, I couldnt find any concrete info other than some got it. I was thrown off because I see other years (earlier) had a check oil light in its place. Mine is always illuminated, it didnt used to be on until maybe couple months ago. Im not concerned because it doesnt make noise and I dont drive the car that much but maybe I should check it just in case.


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[gadget73]

if its constant on, its either oil pressure or water temp. Easy test, unhook the wire off the oil pressure sender and see if its still lit with the key on and engine off. If it is, unhook it from the temp sender. Could also be the wire is chafed and shorting to ground. I would make that work, its not much of an indicator but its better than nothing.

or put in actual gauges since the idiot lights are pretty near to useless.

[87GrandMarq]

It would seem unplugging both had no effect but I was a little rushed. I was driving and when I stepped on it, it felt funny. Then coming to a stop it started to sputter. I shit a brick because of the check engine light then this. I got it in the driveway and popped the hood and found the plug wire on cylinder 6 popped off and was arcing on the exhaust manifold. I plugged it back in and all is well again lol. Ill see if I can get my gauge on there when I have time just to see what the oil pressure is though.


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[87GrandMarq]

Alright so I’m revisiting this because another issue popped up. I replaced the coolant temp sender and all was good for a time. Now recently I’ve noticed when it’s hot and I’m coming to a stop the light will come on and won’t go out until I start driving again. It can idle all day long no problem it’s only when it gets driven. Does this sound like a failing sensor or do you guys think the oil pressure is low? Also where do I hook up a gauge? I believe I need a special tool to remove the oil pressure sensor right?


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[87gtVIC]

You do need a special socket to remove the sensor for the oil pressure. What some here have done is remove the oil gauge sensor and put in a mustang oil pressure extension and then add a gauge or whatever. It brings out he ports for the oil pressure accessory's to attach to away from the oil filter and block. Too tight otherwise to get anything in there..well sending unit wise for gauges.

You would hook up a gauge to the port the sensor is in. Again..may be difficult to tighten anything down due to spacial constraints.

My guess is your oil pressure is low. Next time at a light with left foot on brake pedal and light illuminated try giving it some gas to see if the light goes out with the rpms. I am guessing it would based off your descriptions.

[Tiggie]

These are fairly famous for oil pressure switches that drift out of spec over time.

Get a real gauge on there to check it.

[87GrandMarq]

Thank you guys for the input. It’ll go away when I start accelerating but not immediately, there’s a bit of a lag but yeah it could be low oil pressure. The thing is it doesn’t do it at idle in gear where it’s lowest where you’d think it would come on its only when coming to a stop. My guess is that the aod sticks in 4th and only downshifts when almost to a stop so it drags the rpms down. Welp there’s only one way to find out I guess. I’ll let you guys know what I find. I’m thinking about adding a remote oil adaptor and adding an inline port for the oil pressure gauge.


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[sly]

Also, check the oil level and make sure it's not too low. The oil pickup is in the rear.

[87GrandMarq]

Sounds stupid but I didn’t check. The low oil light has been on for some time though. When it first came on I added a quart and the level looked to be fine but the light didn’t turn off so I just figured it was a faulty oil level sensor but maybe it somehow did lose more oil. Luckily the car hasn’t been driven much recently, I’m afraid of screwing it up. I’ll check it later and probably throw in another quart.


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[sly]

The 88 MGM I used to have would turn the low oil light on at about 2 quarts low, but would go off immediately after adding one quart. I would top it up though. And if you drove around with the low oil light on... you may have been even lower than that and be sucking air when slowing down.

[87GrandMarq]

I added about a quart or so and it turned off. I didn’t have time today to do any testing but I will hopefully later this week. And yeah as soon as it started coming on I took it off the road and only drove it to and from the gas station.


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[gadget73]

Some of these do not have accurate dipsticks. Only way I know to verify is to do an oil change and see where the oil lands on the stick with 5 quarts after draining both pans and replacing the oil filter. It should be in the crosshatch area after startup to charge the filter. I had one from a junkyard car that was way above it and a previous owner had scribed new lines on the back of the stick.

low/no oil pressure at hot idle is also not uncommon but make sure you have enough of the right grade oil first. Confirm whats going on with a gauge. If its under 5 psi at hot idle the switch is going to come on. Basically means the bearings are worn out. The proper fix is rebuild the bottom end, the realistic fix is running slightly heavier oil and just making sure you have the right amount. These things will go for years with loose bottom end bearings as long as you don't rev the piss out of it.