Oil pressure

[miller time]

My dad and I was changing out a 4.0 to a 4.0 no big deal. He got it running and it ran fine blew a rad hose that we forgot to tighten. Well we got home and I watched it a it idle 800 toms or so and then it drops to 500 and then the oil pressure shows nothing for 5-15 sec and idle back at 800 and reads again

 

[Spott]

And then what? If the rpm's dropped once, ever, you just keep am eye on it.

If it does it regularly, then be concerned. If that's the case, try and figure out if it corresponds to anything else going on.

 

[miller time]

It does it every time I rev it up

 

[Earl43P]

Low rpm like that does make the oil pressure gage drop to zero because the oil pressure pressure switch is about 8 psi. Low idle will trip it to a zero psi indication = NORMAL at THAT low of an idle.

Focus on the low idle problem.
The IAC controls idle.
A simple test is to disconnect the IAC and see if idle drops. If it does, the IAC is at least working. That doesn't mean it isn't STICKING.

With your newly installed engine, you may also need to let it relearn its idle strategy.
Let it idle for 4 minutes in each of the following conditions:
Engine must be at operating temperature:

Park
Drive, foot on brake pedal the whole time.
Park with AC on
Drive, foot on the brake pedal the whole time, AC on.

If it has a clutch, you only have to let it idle with foot on the brake and again with the AC on.
It's best to do this after resetting the PCM (Disconnect battery, touch the removed Negative cable to the Pos Battery post still connected to the battery. Or simply turn on the headlamps with the battery disconnected. Either method works).

Another possibility is that you have a vacuum leak and the PCM is trying to compensate for it. Make sure the tube from the air filter box to the throttle body is tight and has no holes in it!

You can clean the IAC shaft with a solvent and then whack it against your palm so the plunger thwocks as it slides along the shaft. Again, the IAC contols idle by metering a certain amount of air to maintain idle. Another thing to check is that the throttle body plate is fully closing and clean. Do not adjust that unless you are going to probe the TPS center wire for proper voltage with the plate closed, less than 1 volt. I think .96v is optimum, but you should look up the TPS test and do that.

 

[miller time]

Ok we changed the iac from the donor truck that ran fine before pulled and unhooked the battery and did the steps
It now high idles more that it low idles and drops oil pressure shifts crazy like you have to let your foot off to shift and it over heats but we are going to change thrumstat and clutch fan

 

[scotts90ranger]

I would check for missed grounds too.

 

[JackDW]

I would check for missed grounds too.

+1 and hopefully you didn't make the same mistake I made. Painted the frame and under the hood and then connected a ground. Paint made a nice insulator. All kinds of funny electrical issues. Removed the ground and scraped down to the metal and reattached which fixed a lot of funny little issues.