Questions on start voltages cold vs warm

What's the min voltage the computer needs to see? 11 volts? If you want the full story read below.

So in trying to troubleshoot a problem with my 00 xj 4.0, Ive been looking at my voltage during starting. When its cold say below 30 degrees and the engine is cold or partially warmed up Im getting a reading of about 10-10.5 volts while cranking. Goes normal, 14 ish as soon it is running. I had tried this with the motor cold and then while idling and say warmed up around 150 ish the results were the same. I used a load tester on the battery and it tested fine.

Today I tried the same thing with the outside temps in the upper 40's and with the motor at 210. I got a reading of about 11.2 while cranking. Im thinking the starter may be pulling excess current and resetting my computer. Either that or the alt is not putting out full juice while cranking in the cold. I plan on pulling and having them tested but may have to try them cold, maybe put them in the freezer or wait till its cold enough outside. Also I do plan on retesting when its really cold out and the motor is up to temp all the way.

Starter-solenoids are volts stupid and amps happy. Engine management systems are volts happy and amps stupid. If the available voltage drops below 9.0 volts the engine management system will start to act wonky.

Measure the voltage between the battery's terminals when cranking. Post up what you get.

NOW, with that said, what is the issue you are trying to resolve? You think your computer is resetting? That would essentially require a complete disconnect of the battery.

The results I posted above were between the battery terminal with a meter. I will try this afternoon again as the temps are in the 50s today and Ill do it with the motor cold this time.

The problem Joe, you may remember, was in order to cold start my xj I had to press the pedal about a 1/8-1/4 and hold the throttle for about 30 sec till it idles on its own. Once its at operating temp it will restart fine, idle is a bit low at first but kicks up right away.

Im checking cranking voltage/battery/starter/alt because it was mentioned that maybe the starter might be pulling to much power and dropping the bat voltage causing the computer to reset forgetting idle settings. But it sounds like thats not the case if 9 volts is the lower limit.

I tested the TPS yesterday and it looks like it is bad, so if that doesnt fix it then on to the next thing.

Ok.

If you are having to hold the throttle open that is only accomplishing one or two things. First, it is changing the data the TPS sends the PCM. Second, it is changing the air intake--essentially effecting what the IAC should be doing.

Your voltages are reading fine and I wouldn't give that issue anymore thought.

I would pull the IAC and clean the throttle body thoroughly, paying particular attention to the IAC port. Clean the IAC pintle, but be gentle it is plastic and can break.

If the TPS is out of spec replace it.

Post up what you find.

joe_peters said:

Ok.

If you are having to hold the throttle open that is only accomplishing one or two things. First, it is changing the data the TPS sends the PCM. Second, it is changing the air intake--essentially effecting what the IAC should be doing.

Your voltages are reading fine and I wouldn't give that issue anymore thought.

I would pull the IAC and clean the throttle body thoroughly, paying particular attention to the IAC port. Clean the IAC pintle, but be gentle it is plastic and can break.

If the TPS is out of spec replace it.

Post up what you find.

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If the battery is above 9V while cold cranking it's OK.

What Joe says is entire correct. Be very cautious with the IAC pintle, DO NOT energize it outside of the housing, and don't try to take the rubber/plastic cap off. Be careful not to turn it as well, as they tend to "conform" to the bore and get stuck when you turn them.

milehigh said:

....in order to cold start my xj I had to press the pedal about a 1/8-1/4 and hold the throttle for about 30 sec till it idles on its own......

Click to expand...

This is a common symptom of poor wire connections at the battery or a failed battery.

Low battery voltage re-boots the PCM and idle settings are deleted. You get the same symptoms (and re-boot) when you leave the lights on and run down the battery or when the battery is disconnected for an extended period of time.

Have the battery Load Tested at the auto parts store, bring money for a new battery.

If you test less than 12 volts across the battery terminals after the engine has been off for 10 minutes, your battery is weak.

Tim_MN said:

This is a common symptom of poor wire connections at the battery or a failed battery.

Low battery voltage re-boots the PCM and idle settings are deleted. You get the same symptoms (and re-boot) when you leave the lights on and run down the battery or when the battery is disconnected for an extended period of time.

Have the battery Load Tested at the auto parts store, bring money for a new battery.

If you test less than 12 volts across the battery terminals after the engine has been off for 10 minutes, your battery is weak.

Click to expand...

Please read all the way through people's posts before responding. Very annoying to have to repeat one's self. I stated above what my voltages were and that I had load tested the battery.

Joe if I had a link to the previous posts I would link it. But in summary I have checked all terminals, connections, gnd points, IAC cleaned and replaced, and voltages/battery confirmed good, thanks by the way.

I replaced the TPS tonight and it fired right up and idles much better, but the motor was up to operating temp. I will have to see how she does in cold weather to confirm for sure. One strange thing was that the TPS tested close to the same as the old one. There is a post currently going for this, but here is the skinny. Old .75 idle 3.8 WOT, New .95 idle 3.9 WOT. I will have to retest after the computer takes a reset and gets some miles on it. It shouldnt change the values unless the 5v coming into the sensor is some how controlled by the computer.

This may be the root cause for the original problem which was a code of P0505 I think? Idle circuit error. The recent problem of not idling occurred a month or two after the light came on and stayed on. I cleared the code today and after a few on off cycles its staying off. We shall see if it stays that way. I thought I had the problem fixed last time when I replaced the IAC, but it came back about a week later, the code that is. Time will tell. Thanks again for the help. Oh by the way the FSM says that code is caused by the computer. I hope to hell that isnt the case. Another oh by the way is the idle problem did pop any other codes aside from the same P0505.

You described the same symptoms about three weeks ago in this thread http://www.naxja.org/forum/showthread.php?t=1051184 and I suggested the same thing. Your battery is probably failed.

If you don't want any advice, you can simple ignore a post, there is no need to be arse when people are trying to help you.

The TPS is fed reference voltage from the ECU.

The voltage isn't the whole story on a TPS, if it's got a dead spot it will do wonky things.

Why did you not continue your other thread, best to keep all things suggested and tried in the same thread so things don't get repeated.

It is still possible that your battery is failing, or that your cables are trash, but generally if the battery reads over 9 volts while cranking it's OK.

When you cleaned terminals did you inspect the wire back into it's insulation? If it's black copper under the insulation then the cable needs replacing.

Tim I responded like an ass because I had described above that I had already tried those things. So unless there is another way to test a battery than when its cold using a load tester or while cranking the motor in cold weather Im all ears.

I didnt continue because I wanted a specific answer to specific question. I know I should have left out all the back story to avoid this. My mistake I will learn to be more to the point in the future. Can you elaborate on the battery? If I have tested as I described above then how else would you test it? Or can you test it? Would I just have to assume that it is bad?

No I didnt inspect the wire under the insulation, didnt think of that. If memory serves me right I checked my voltage at the fuse block as well. But that is a good idea I could test cranking voltage back at the fuse block. I think I ohmed the cable but will do so again to cover my bases. Im honestly starting to think that I have a junk PCM. I do still need to check for a spread pin at the computer.

The problem did reoccur this morning so it is definetly dependant on the motor being cold. I may also pull the computer and leave it in the house over night to see if there is a cold solder joint in the computer. Ill have to recheck the 5v going to the TPS under different conditions.

voltage /= Amperage

A nasty cable can still show full voltage, but not be capable of providing enough amperage to actually run the device.

Conversely if it's ground stuff the voltage potential will get all jacked up.

Cut the insulation back just a bit on your mains cables, if it's looking black in there your cables have the corrosion cancer.