Hello and thank you for all the answers from my last question. Recently, my car has started to run rough with the check engine light on. I cycled the key 3 times and checked for codes. I got a code for the MAP sensor and a code for the 02 sensor. I got no codes for a misfire, so I guess the plugs and wires are okay. I bought a new MOPAR Map sensor and installed it. It still runs the same and the check engine light is still on… My question is, is it necessary to disconnect the negative on the battery when doing this and for how long? I did disconnect it for about 2 minutes and it still was the same. Any thoughts on this would be greatly appreciated. Thank you. John
Disconnecting for a couple minutes should be enough to re-set the computer. If the light comes on again, it means the fault is still present and continuous. The problem with codes is that it only tells you that the computer saw something that was outside of the normal readings and flagged it as a problem. What the real problem is may be different and causing the readings to be off, or it could be anything in that circuit such as a connection or wiring. I suspect it may be a too rich or too lean condition that could be throwing the o2 off because it can’t adjust anymore. A bad MAF sensor could do that but hopefully one of the pros will chime in. (Never had a bad MAP though in a million miles although the code did come up a few times and just ignored it.)
If you got a code for a bad MAP sensor, I expect you probably had a bad one. It measures the intake manifold pressure (vacuum actually, a negative pressure) and when the engine is idling the ECM knows what the vacuum should be. Unless you had severe vacuum leaks, the sensor probably was bad. And that could contribute to making the ECM think the O2 sensor was bad, as the MAP sensor reading is used to calculate the air/fuel mixture.
hmmm … well, I guess if this were my car the first thing I’d do, on a 89 I expect this should work, I’d disconnect the battery for a couple hours, maybe overnight, reconnect and see if that would reset the ECM enough to turn the CEL off. If it remains on, read the codes again. Maybe you’ve got a bad O2 sensor too. If the MAP codes remains, measure the intake manifold vacuum with the mechanic’s own gauge, maybe something remains wrong there. A plugged cat for example can cause the vacuum to decrease, which might could fool the ECM into thinking something is wrong w/the MAP, when it is really the cat that is the problem. Is the car otherwise idling and running ok, same as before? When you ask for max acceleration, like when coming up to speed entering a freeway, does it deliver as it used to?
He said it runs rough yet so something else is wrong. I don’t want to lead anyone astray but the readings from the O2 are not used until the engine warms up enough to go into closed loop. So if it runs rough at start up and also after warming up, I’d suspect something other than the O2. If it runs OK cold but rough after warm up, might be worth changing out the O2-should be cheap.
Have you checked for a vacuum leak?
PS–I would still pull the plugs and check them. It’s easy and doesn’t cost anything. Just because you have no misfire code doesn’t mean the plugs and wires are OK. Check the wires too.
Also check that you have manifold vacuum at the MAP sensor. I experienced a MAP sensor code. When I checked for vacuum at the sensor, there was none. When I pulled on the vacuum line heading under the manifold (I assume you have the 2.5 liter 4), two feet of it came out. The break was in the hard plastic tubing portion. Cheap fix.
Good point @Researcher, best to check the vacuum right at the MAP sensor, not at a different vacuum port. It’s also possible the replacement MAP sensor is faulty.