My 89 IROC-Z 350 has recently been running odd. It has a nice high idle and the oil pressure was usually around 40 psi. But recently the idle has been rather low and the psi keeps dropping below 30. then when I gas it the psi goes up to 40 as the rpms increase past 1500 where it used to idle. I checked my oil level and it was well into operating range. I am gonna change the oil soon but I am wondering if anyone knows what is causing this. there is no performance loss but I just don't like the oil pressure being so low. I am using SAE 5W-30 Valvoline Max-Life oil. Would more viscous oil resolve this problem? Please help me out

 

The pressure sounds fine, 20psi at idle comes to mind from the manual. 5W-30 is kinda thin to use in a well-used engine, I prefer 10W-30 in Summer and 5W-30 in Winter.

 

I was drag racing on the weekend and sport shifted, starting in second, do you think it could be a computer issue? I checked my oil levels and they were just fine. I left it in drive all day today and it seemed to idle higher. I guess it could be a computer control activity in the transmission. thanks

 

Usually a higher-than-normal idle is caused by a vacuum leak, nothing to do w/ your oil pressure but worth diagnosing. Check all vacuum hoses for cracks or damage, make sure they're all connected, and listen for hissing around them while the engine is running.

As for the oil, again 5W-30 is a tad thin for Summer use in a high-mileage engine in my experience.

 

0-60,

The target idle speed is controlled by the IAC motor. If the throttle body and IAC passages are clean, the TPS is set correctly, and the throttle shaft is not worn or binding, the idle should be controlled accurately at the speed desired by the ECM. The target idle speed is determined by the coolant temperature, so a reliable CTS is important.

As for the oil pressure at idle, it will vary a bit depending on the idle speed. Lower speed will produce lower pressure. Idle speed above a certain point (where the pump is beginning to open the relief valve) will produce a more consistent pressure. Thinner oil will make this idle point higher, of course.

I'll second the vote for a little more viscous oil above 40°F. Personally, I don't use 5W-anything even in -40°F temperatures, but that's just me. Of course, I cheat by using synthetics.

 

Could it be a blown head gasket?

 

Yes, absolutely, now go pull both heads and swap gaskets. And if that doesn't fix it, the whole engine is shot. And if that doesn't fix it, the wiring harness is bad. And if that doesn't fix it, the ECM is shot. And if that doesn't fix it....

(or you could do some troubleshooting instead)

 

come on kevin why would anyone want to actually trouble shoot anything when you can asak on here and people can guess at the problem.

 

0-60,

I think the point on the table is that it would be a good idea to do some basic investigation. Check the ignition system to make sure it is reliable. Make sure the fuel system is operating correctly, with a good fuel filter and correct fuel pressure. Make sure the throttle body is clean and the TPS is set. These are basically no-cost procedures that should be done anyway to eliminate the variables and as a part of general maintenance.

If you suspect a head gasket leak, there should also be a more consistent misfire and possible loss of coolant into the oil. But instead of guessing, it would be relatively easy to perform a compression test or cylinder leakage test to verify the guess or elimintate it.

 

When I start up the engine it starts up right away and the idle sits were it normally does around 1200 rpm, but after a little bit of driving, it drops to 750 rpm, occasionally it dips into 500 rpm, and the oil pressures goes far down, but when I gas it, it flies back up to 40 psi where it should be. Could an alternator that is too small for the engine be a problem? when it's idling really low, the volt meter seems to read a little less that 13V, but it's a new battery and when we put it in it was staying up nicely at 14V. it's almost as if I can't have it idling or it will act up. I'm gonna change the plugs, clean out the throttlebody, and change the oil this weekend and see if that gets results. I'll also get a diagnostic done at autozone
thanx