1998 Caravan 3.0 litre
215,000 Kms (134,000 miles)

I've been having problems with a check-engine light (Misfire). While going through the possibilities, I decided to check the MAP Sensor, as according to various newsgroups, this can be a cause.

The instructions for checking the MAP sensor on the 3.0 can be found at:http://www.autozone.com/az/cds/en_us...rInfoPages.htm

I am confused though. The instructions are to measure the voltages at the Sensor Connector. No problem there, but I'm a bit confused. How can you tell that the sensor itself is bad by checking the voltage at a connector which is disconnected from the unit itself, withnothing between the two?

In any case, although I could read supply voltage, I couldn't get any voltage between pins A & B (Sensor Output Voltage) with the key "on". All other measurements were good. I replaced theMAP Sensor, re-connected the batteryand got the necessary 5 volts between pins A & B. When starting the engine, I could detect a change in the way the engine was acting and took it for a test drive on the highway, as the check-engine light normally comes on at higher speeds (80 k/hr or faster).

Lo and behold, the check engine lightremained off. I was actually starting to smile when, after40 kms and 5 kms from the house, the check-engine light came back on.

When I returned to the house, I again couldn't get any voltage between the same two MAP Sensor Connector terminals, albeit with a new sensor.

Any help would be greatly appreciated!

 

What codes do you have ? Because if its a misfire code, it could be a spark or wire. Or even a cap and rotor too.

 

Hi.
The self-diagnostic code is 43. Had it scanned, and it came up as a misfire in cylinder #5. I've already changed outthe distributor cap, rotor, plugs, wires, air filter, PCV Valve and many vacuum lines (but not all yet). Checked for spark from the plug at cylinder 5. No probem there, although that was in park. The "Check Engine" seems to be detected at speeds above 80k/hr.
I'm also getting a "ticking" at all RPM's, but don't know whether it's a valve or exhaust manifold leak.
The problems described above with the two MAP sensors is baffling me.

 

need to check vaccuum lines in the intake manifold.. cyl. 5 misfire on the 3.0 it is very very VERY common to have a vaccuum line plugged INSIDE the intake.. normally near the egr valve... check it out.. also check to make sure the vaccuum lines are connected .. which is probably not the case ... the new map sensor corrected the COMPUTER.. but not the problem.. the computer tells the engine what to do.. if the map sensor is at fault.. it doesnt deliver t he right fuel/air misture.. check the vaccuum lines.. can of carb cleaner.. run the engine.. start spraying it on the vaccuum lines.. when the engine idle smooths .. you will have a vaccuum leak in that area.. find it and fix it.. but i am betting its not the line.. its the internal port next to the egr valve..

 

Master Tech,
Excellent suggestions. Thanks very much.
I'll give it a shot and post the results.

 

Interesting update.
Replaced most of the vacuum lines, with no result so far.

Reset the "Check Engine" light (negative battery terminal removed for a few minutes) and just finished a 4 1/2 hour highway run. During the whole day, I noted the following:
1. Check engine light came on again at exactly 110 km/hr
2. The check engine light would "blink" as soon as vehicle speed reached 110 km/hr for any more than 5 seconds.
3. Light would always go back to steady as soon as I lowered the speed below 110 km/hr.
4. When going up long hills, even though I would increase throttle to stay at say 105 km/hr, check engine light would remain steady.
5. However, when going down hills, even with less throttle, check engine light would again blink when speed reached 110 km/hr.

Question - How is a misfirelinked to vehicle speed and not throttle position, etc? Or is it?

Thanks in advance!