Jeepwalker

Hey guys. Have a morning start-up issue on a buddy's 2001 Cherokee that started a while ago. It's experiencing high rpm on morning cold start-up (2000-ish RPM). Only in the morning, even in the summer. Engine immediately settles down to fast idle, then normal idle like it should. It's just the racing on AM start-up we're concerned about. During the day it starts at normally, even if the vehicle sits several hours (like during work).

If we 'bump' the engine over before actual starting in the morning (just a quick engine 'bump'), it starts and idles normally (no engine racing). Which is wierd. No codes or scanner readings which look out of place. I had cleaned the throttle body and IAC a while ago, didn't make any difference. Didn't see any removed or damaged vacuum or PCV hoses. I guess I should look at the TPS during am start-up ..which I haven't done yet.

Your thoughts??

Thx

agreen

I believe it's a feature for catalytic converter warmup. Mine does it too. It's on cold mornings... maybe below 45F? I've never really checked the temp at which it does it though.

Jeepwalker

Thanks for the reply. It spikes (revs) way up, for a split second, then immediately drops down to high idle which is where it would normally start up at ...then once warmed up a bit, goes down where it should. Not normal. It could be dangerous to the engine if it does that some am at -15.

I went out this am, and checked things on the scanner before starting or touching anything (not even the pedal). The TPS was at .53 and temp sensor looked good. For some strange reason it didn't spike high today like usual (my luck). But looking at the TPS and other settings in the scanner once running and afterwards, I didn't see anything terribly unusual, no drop-outs on the TPS that I detected. That's what's had me scratching my head on this one. Then again, I'd probably need to hook up direct to the TPS with a fast VOM to see 'instant' readings.

I'm wondering if it could be a mass air sensor? Not reading the initial air flowing through in the am. But you'd think there'd be a code for that.

Thanks for the ideas.. keep them coming! (please)

agreen

There isn't a MAF sensor.

Here's how I found out my TPS was bad:

Using an ELM327 scanner and an app called "car scanner" I chose RPM and TPS for my graph. You can only choose 2 with the free version. Anyway, you can see that initially while driving the TPS would read down to zero, then eventually it got to the point where it wouldn't go below 15%. This gave me a weird high idle. I would test and test and test the TPS, and each time I got good voltages. Seems like it would test fine in the driveway, but go bad while driving. Yours might possibly be doing something similar. Not the exact same, but a similar failure.

Jeepwalker

Thanks for that. I'll set up graphing tomorrow on my scanner. Not sure if it's as quick as what you're using. Thanks for the tip on the app. What cable are you using?

Thx

agreen

It's Bluetooth. Buy one from Ebay or Amazon or whatever for like 10-15 bucks, download the app, and pair it to your phone. It does... so much for diagnostics. I've heard there are better apps you can pay for (like another $10) and it'll do as much as a $5k scan tool. I only ever use mine for reading sensor data and codes, so I only use the free app.

Torque (lite) has a better dashboard. You can choose several parameters to watch at a time.
Car Scanner has a better datalog feature.