I had the engine totally rebuilt on my '91 Montero last summer, and needless to say ended up having to fix a LOT more than that. My current issues are:

1: Constant low oil pressure, which makes the engine sound like hell when it's running. It has a new oil pump, oil gets changed every 2k miles (not a daily driver, and here in AZ the oil just always seems to get dirty fast), and the oil filter is always replaced. The first mechanic I ever took it to told me that "Monteros just run on low pressure" and that it was nothing to worry about...and then he proceeded to take a chunk out of my crankshaft when doing the heads, not tell me about it, and cause me to need the rebuild. So needless to say, not sure if he was just blowing smoke up my behind or what. Either way, it runs loud, and I don't like it/worry it's going to eventually cause internal damage.

2: Idle seeking. When in park or neutral, the idle seeks up and down, up and down. I had the MAS rebuilt, and replaced the IAC valve and TPS. The only other thing I can think to replace is the ECM, but I'm hesitant to invest the money if it isn't going to fix the issue.

The mechanic I've recently been taking it to says it's low oil pressure causing all of the noise, and that it's timed correctly so they can't fix the seeking. Given their lack of solutions I guess it's time to yet again look for a new shop. Any suggestions on what I can look to fix to solve these issue that I haven't already?

 

Amandarenee, the story with the low oil pressure being normal does not sound right at all. I would be VERY concerned about that issue. I do not have specs for your exact model year Montero, but on a little later models (year 2000+), the oil pressure at idle is supposed to be above 4PSI and at 3500 RPM normal value is 43-100 PSI. These engines use oil pressure to maintain valve clearances, so running low pressure in the system would result in valve slap noise. Additionally, low pressure would result in insufficient lubrication of the vital components.

You said the engine was rebuild (crank shaft reground, or something along those lines.). Do you have any information on what size main bearings and rod bearings were used upon re-assembly? If these are not sized right, the excessive gap would allow oil to escape too easily and that is why the pump can't build up pressure. You can run a pressure test on the oil pump. Take out the pressure switch and plug in a pressure gauge instead. Run engine and when fully warmed up, test oil pressure at idle and at higher RPMs. If below specified - you have a problem. You may need to pull the engine and check oil clearances on all rod bearings and on all main bearings. If found to be excessive (standard value: 0.02-0.04mm (0.0008-0.0016"), limit: 0.1mm (0.003") - you need different thickness bearings.

I would not even worry about idle until you get your oil pressure situation sorted out.
Just as a thought - if your oil pressure is found to be within normal range, then the engine noise issue might be related to air trapped in valve lash adjusters. You will need to preform VLA bleeding procedure. Search this forum, there are several links to the pages in manual on how to properly do it. But ONLY do this procedure if your oil pressure is normal. Keep us posted.

 

Following bc I have the same issue on the same year.