Would someone check my MAP math?

[Jason]

A little background, recently the Jeep's been shutting down at speed. A click off of the igintion key and I drive away. Well, yesterday it shut down on me twice while on the freeway. I decided to take the streets home and just before I was home I got a few backfires before she stalled again. Sounds timing related to me. With a few clicks of the key, I got the code and my MAP sensor numbers came up.
Here begins my fuzzy math;
The MAP sensor informs the computer about my air/fuel mixture correct? Now, if I have a vacumn leak, would that cause a faulty MAP signal as it is connected to the intake or do the sensors just die on their own?
If I disconnect the sensor, the Jeep should run better, the same, or worse. If the first two, replace the MAP? If worse, time to move on to the next sensor. TPS, CPS, IRS...

I did replace my fuel filter over the weekend thinking that my problems might be fuel related. I couldn't remember when the last time I had changed that. No change there. I did read some older posts and for some of you that had what appeared to be ignition related problems were fuel pumps going bad. The tank is almost empty so maybe time to drop that and have a peak.
The Jeep is a '95 with 130k on the engine.
thanks.

[Jeeperator]

MAP stands for manifold absolute pressure so no it doesnt inform the computer about air and fuel just air pressure or how much vacuum the engine has in the intake manifold. The O2 sensor reads air and fuel mixture. Backfire is excessive fuel in the exuast so if its doing that I doubt the fuel pump is going out. Sound ignition related to me. I would get it scaned and see what info the map and other sensors are putting out while its running before replacing parts.

[aw12345]

Jason, what it comes down to is narrowing down the cause to either fuel related or ignition related. One of the basic parts that fails is the crank sensor run it try icing it or heating it up. Still start with the first part if need be hook a auxiliary tach to the coil to see if the ignition signal to the coil goes away when it dies.

[6spdYJ]

Lots of good tech here, backfiring and engine quitting etc. http://bc4x4.com/faqs/yj.cfm?cat=5&faqid=38#faqtop The MAP sends manifold absolute pressure (or lack thereof - vacuum) to the ECU. If it sees vacuum loss, it thinks the trottle is wide open and will try to dump more fuel. This will cause backfire and loss of power. Look at your plugs to see if you're burning too rich. Check your codes to make sure there isn't a Throttle Position Sensor fault too. That signal should be a fail-safe against this condition, but it may not be programmed that way. Not sure there.

[Jeeperator]

Jason said its throwing a map sensor code fault, he needs to start there first and find out why its doing that. Snapon scanner hooked up will display values of most the sensors while your driving it down the road. I had a tps sensor go bad and intermitently worked at different throttle positions a map might do the same thing if its bad. Whats that phrase KISS (Keep It Simple Stupid) LOL.

[aw12345]

If it has an active map sensor code it's time to see which one of the 2 codes did it set.



Once that is established check it with a scanner or a digital volt meter. Voltage at idle should be somewhere between 0.9-1.5 volts.
Snapping the throttle open it should move quickly towards 4 volts or so. Better test is to use a vacuum hand pump to manipulate it.
A bad or worn fuel pump can cause the map sensor voltage high code

[Jason]

Thanks Art. Are those codes for a TJ and above or just Jeep in general?
I think I need to start with the MAP sensor. I done some reading on how to test the sensor and all I need now is vacuum hand pump. To the parts store :yay:

Time to kill the ignition gremilins.

[aw12345]

Those are YJ specific codes

[Jason]

Well the vacuum checks out. The code being thrown is 14. Which by the above is electical. So I will check the voltage but I'm begining to think that I just may need a new MAP sensor. $20-$30 at Autozone and I should be golden again. I've also heard that if you get dirt and other crud in front of the TPS ?, the one by the transmission, it can act quirky as well.

[aw12345]

code 14 means it either sees a very high vacuum or most likely the sensor is kaput, the only way to know for sure is to see if one of the wires to it has 5 volts another one has a good ground, meaning no resistance to ground, then the third wire should see about 1-1.3 voltas at idle and around 4 when you snap the throttle

[Jeeperator]

I've also heard that if you get dirt and other crud in front of the TPS ?, the one by the transmission, it can act quirky as well.

Do you mean CPS (Cranksaft Position Sensor)?. TPS (Throttle Position Sensor) is on the throttle body.

[Jason]

Yes, CPS (Child Protective Services) it is. I just couldn't remember at the time.

Update.., MAP checks out. Voltage, ground, & vaccum. Time to disconnect the battery to rest the codes, drive the Jeep around, and see if I get a code again.

[Jason]

After changing the MAP, cap n rotor, new plugs and wires everything seemed to check out. Took the Jeep out and it died on me once but then I remembered that I was on fumes. Filled it up and it seemed to drive better than it had for a while.:yay: Fixed!

Fast forward to HDR....
Standing next to the Jeep talking about my ignition gremlins, the person I was talking too said "You got a vacuum leak!" Engine was running and he heard the familiar hiss. Found and fixed the leak and all seemed well.

Out on Roger's sightseeing tour and the check engine light comes back on. No code is given at all. It did this all weekend with only one engine hiccup that I can recall.

The engine light with no code bothers me. It's on for a reason and I want to find the reason and fix it. Hopefully with out replacing every single sensor.
Just for kicks, I'm going to borrow a fuel pressure gauge from Auto-zone, maybe drop the tank and inspect the pump.

Any other ideas I need to check. Timing? I like bring out my timing gun every now and then. It's fun in the dark.

What could cause the check engine light but not register a code?

[aw12345]

Bad PCM would do that

[6spdYJ]

Make sure it's check engine or service required. YJ's had some egghead program a maintenance required light to fire off every 42,000 miles or so. '87-'91 can be reset with a paperclip, but later models have to be cleared with an OBDI programmer. The maint req'd light doesn't throw a code. Disconnect the battery and reconnect it. A CEL will not appear until the engine is running. This stupid maint light is based on a milege trigger in the ECU and will come right back.