Map sensor question

Hi,
I suspect my MAP sensor is failing on my 93 4.0 L and wanted to know if I could unplug it's electrical connector but leave the vacum line connected and drive around to see if it makes any difference. Will this cause any damage to the engine computer? or will I just get a check engine light come on? The symptoms are it cuts out sometimes when changing from open loop to closed loop, runs ok without any sensors, just that idle speed is a bit higher.

Thanks for your help

The MAP sensor provides data for fuel metering - unplugging it will probably get you a MIL and trip the "limp home" mode - power sucks, fuel consumption sucks, but it can get you home.

You should be able to score a MAP sensor for not too many bucks (I've seen performance units in the $30-40 range - two-bar and three-bar for forced induction!) so scout around a bit. Stick to Borg-Warner, Bosch, or ACDelco for good units.

5-90

The stock MAP sensor is a GM unit so you should be able to find a GM 1-bar sensor cheaply. If you disconnect the connector from the MAP sensor, the "check engine" light will come on and the engine will go into "limp home" mode with the TPS taking on the main role as a load sensor. The engine will run but not very well.

Hi,
Thanks for your reply's, I forgot to mention ( I cannot remember everything at my age) that I had disconnected the Water Temp sensor, by the T stat. The computer now stays in closed mode or (is it open) all the time and it does not cut out, it runs fine, just idles a bit higher. What I wanted to try was to reconnect the Water Temp sensor but cut out the MAP and see what happens. The next move would then be to try it without the Oxygen sensor connected and see if it still cuts out when switching from closed to open mode. I am trying to run a big car (4 Litre) on a small pension, which is why I cannot run out and buy all new sensors! which would probably solve the problem.

Thanks again

I don't know how much help this might be to you, but the XJ sensors tend to follow the same curves and specifications as most General Motors products (Chevvy, Pontiac, Buick, Cadillac, GMC, &c.) and if you have any sort of access to automotive salvage, you might try that for a "band-aid" fix. I've always been a little leery of buying electrics from a salvage yard, but you can usually keep things running for a bit that way until you can get the cash for new parts. Most salvage yards on this side of the pond, you can usually find the sensors and take them out for a few dollars, which makes it usually worth the chance (there's no guarantee that you'll be able to get a working sensor, but it's worth a shot.)

If you have access to a hand-powered vacuum pump, you might take it with you (along with an ohmmeter) and use that to test the sensor. From what I see on wiring diagrams, you should see a change in resistance across terminals A and B as vacuum is applied (I don't know what numbers to expect offhand, but you should see a smooth change from atmospheric pressure to full vaccum. If you really need them, I can check it out on my 88 Tuesday or Wednesday, and let you know what I see...)

5-90