1991 Rodeo Low Idle Low Oil Pressure

[P99]

Good Day, hope you guys can help.
1991 Rodeo, 3.1 rebuilt "professionally" 2,000 Miles ago.
Ran fine for about 1,500 miles, good power, oil pressure Ok, no problems.
Now not so good.
Start up, no problem, starts right up, idles at about 1100, oil press 4 -6x100KPA at idle
First 10 Mins of driving, warms up fine, idle drops to about 750, oil press 2 - 3x100Kpa at idle, idles fine, power is good, all normal.
After about 15 mins of driving, idle at about 700 and oil pressure at 2x100Kpa, all seems normal.
After about 20Mins of driving and come to stop light, rpm drops to about 650, oil press drops to about 1x100KPA, oil light comes on, headllights flicker or go out, buzzing sound coming from drivers side of engine compartment. Engine is running smoothly, doesn't stall.
Turn off engine and restart
Starts fine, oil press normal, at idle revs are about 750, all ok. Give it a bit of gas and let off, oil press drops to back to 1x100Kpa, idle drops down to about 650. Give it a bit of gas and raise rpm up to 700 and all ok. Drives no problem, power good, all normal, come to a stop and idle drops, oil pressure drops, lights go out.

1st time back at the shop that rebuilt the motor. "Did some adjustments, all is good you should have no problems now". 20mins of driving same problem
2nd time back at the shop that rebuilt the motor. "You need a new ICA Valve, we installed a used one that checks out fine, you will have no problems, all Ok, that will be $140.00" 20 mins later driving home, same problem.
3rd day back at the shop is tomorrow!

Help, any ideas.

Any suggestions that you may have on what is causing this problem would be most appreciated. Would be nice to have a bit of an idea what it could be before I go back for the third time and the shop says you need this or that.

Thanks in advance, P99

[Ramblin Fever]

Take a good look at the health of your alternator and serpentine belt, my '91 Rodeo did this just before the alternator froze up causing the serpentine belt to actually catch on fire.

[P99]

Thank you very much Ramblin Fever, info muc appreciated.
Rodeo is back in the shop this morning, and have asked them to check the alternator. Awaiting the results.
So far, they are telling me that I need a new Oil Pressure Sending Unit (old one is causing the oil pressure to drop) and a new Headlight relay (old one is buzzing when it heats up? and that is causing the headlights to dim, they can't tell me why the buzzing stops when the revs go up)
Plan to be back at the shop in an hour or two and will go for a test drive with the mechanic. Fingers crossed that they have no found the fault and fixed it. Thought that even with a 91 they could just plug it into a computer and they would be told what the problem is. For a shop that has been business for more than 40 yrs not too impressed with thier diagnositc capabilities.

Thanks again, will let you know. P99

[P99]

Good Day,

Ok third time back in the shop.
- Oil Press Sender replaced
- Buzzing noise is the HeadLight Relay for daytime running lights.

Drove the car after the new Oil Press Sender installed. After about 20mins of driving, when at idle, 650 RPM, the oil press drops to 100 KPA/14.5 PSI, but the oil warning light no longer comes on, so I guess thats good. Buzzing from relay is still there. Increase the revs to 700 rpm and buzzing sounds goes away and headlights are bright rather than flickering. Turn on headlights and no problem work fine. Only when on daytime running do they flicker when idleing.

Now plan to replace the Head Light Relay, if any one knows the GM Part number for the relay it would be most appreciated. The shop did a search and they could not find a new Isuzu one in Canada. Expect that there is a GM Part that is the same.

Also wondering if oil pressure at idle of 100 KPA/14.5 PSI is normal. Before the rebuild remember that it was always low at idle but don't recall it being that low. Goes up to normal range, 4 - 6 KPA when driving.

Any suggestions most appreciated.

Brgds, P99

[Ramblin Fever]

Did you get the alternator checked under load?

Did your truck come with factory daytime running lights?? My '91 Rodeo did not, if it's an aftermarket feature, there could be a short in the wiring.

'91 year was before the OBDII diagnostic code read-outs, that year is OBDI, IIRC

The lights being dim, then brighter when revs come up is generally a tall-tell sign of the 1st generation Rodeo's of a failing alternator.

[P99]

Good Day Ramblin Fever

Thanks for your message. The shop said that they did check the alternator and that it was ok and that it was the day time running lights which was the problem. Since it is Canadian car it came with always on daytime running lights. When the lights were flickering I pushed the head light button and the head lights came on bright and strong, turned off and the flickering returned. So do think it is just the relay, lets hope. Found an aftermarket relay on the web for $23.00 that says it will work for this application, will order it and let you know.

Thanks again for your help. By the way do you have any pics of your truck?

Brgds, P99

[Ramblin Fever]

Sure do....

[P99]

Thanks Ramblin Fever

Your truck looks great. Inspiration to get going on mine.

Nice truck, if mine ever looks that good I'll send you some pics.

Thanks again for your help.

Brgds, P99

[Philscbx]

Upgrade the oil, it seems too light duty.
Try a day or 2 with 20/50. It's all I use.
In frigid cold as it was here to -25 below zero F, i'll go to 10/40.
They maybe using a very light weight break in oil.
In the end, I'd install a T in the sensor fitting to use a real oil pressure gauge besides the oil light indicator.

[Philscbx]

I like your roof rack, as I could use one on my Amigo.