I bought a 97 softtop 4cyl last week for $2500 with the description of, "it dont start". Well after about an hour I had it started (aftermarket alarm had disabled the key switch). So we go for a test spin, the check engine light is on and at almost exactly 3000 rpm it has a dead miss and will NOT rev above that in gear or out of gear.
Put the code scanner on it and it points me to MAP sensor. Replace it, no change. Check it again it says coolant temp sensor, replace it, no change. New plugs are always a good idea, replace them, no change. Talk to a mechanic he says cam sensor, replace it, no change. Then maybe crank sensor, replace it, no change.
I am getting tired of "replace it, no change". My only saving grace is that I can do all this myself and I'm not paying someone to troubleshoot. So here I am, new to me Jeep that I am excited about and I cant get it up to driving speed.
Help would be greatly appreciated, I want to love my Jeep instead of cuss it!
Thanks for reminding me, always shows P0108 for MAP sensor, occasionally shows P0132 for O2 sensor.
The O2 sensor is rare, maybe 1 in 5 times checking and erasing codes. I read online today that the exhaust manifolds crack easily and cause problems, could the problems I am getting be coming from a cracked manifold in a place that I cant see?
I don't see any O2 sensor issue causing it to not increase in RPM - good idea above about CAT. - if you can remove it, pull it off, see if problem goes away (I haven't looked to see if its easily done, and CEL will definitely be there, but the RPM issue may not.)
I'd chase some wires related to the MAP sensor...something in that circuit is off. (I'm no expert in this area).
Its of course possible that the replacement MAP sensor is bad. but there should be a test for that. IIRC it involves checking the resistance of the part.
FSM would be of great help here.
I need another Jeep...I'm lurking here, waiting for a great deal.
By the way I changed the neutral safety switch last night with of course, no change. I am going to unbolt the exhaust tomorrow and also check the compression on each cylinder tomorrow. I'll post back with results.
This may be a complete shot in the dark. HOWEVER, your first post pointed to your alarm system. If your Jeep has a manual transmission, please ignore the following. Your alarm system may be interfaced with a remote start system. If the system is not working correctly, or has been tampered with, it may still be effecting your ignition system. Just in case you're not familiar with remote start systems, they usually have a few anti-theft triggers that will shut the vehicle down in case someone would try to drive away without the keys. One of the triggers will usually shut the vehicle down when the engine increases RPM's. Since it's already giving you trouble, I would suggest thoroughly removing the aftermarket alarm system. You may want to start with unplugging any harness connections that go directly to the alarm brain. Also please note that a remote start is often a separate module from the main alarm brain. This can usually be found under the dash. The reason I stated to ignore this post if you have a manual transmission is that 99% of the car audio/security shops out there will not install a remote start system on vehicle with manual transmissions for obvious reasons. I used to wholesale a lot of new/used car trades. You'd be surprised at the number of vehicle owners that do not include their aftermarket alarm remotes when they trade in their cars. This can lead to a lot of problems for future owners.
Sorry if I've seemed to ramble. I just wanted to add to your list of considerations.
Today just for fun and to eliminate possibilities I checked the compression in each cyl and they are all nearly identical. So that was encoraging. I then also took the exhaust off rught behind the exhaust manifold and started it with no pipes, cat, or muffler to slow downward flow. And again, no change!
I would put good $$ that I removed all the alarm at the very first, but at this point I am willing to check it again. Like you said, when I got the Jeep it didnt have the alarm keypad. I cut out 2 boxes under the dash and spliced the factory wires back together. I'd bet that it is all out but I am headed to the garage to make sure.
Like I said before it runs "ok" (not rough, but not new car smooth) up to 3000 then has a hard, dead miss. And then will not rev any further in any gear or park or neutral.