Odd Mi16 Oil Pressure Issue

[Anthony 1,003]

A friend of mine has a 1.9 Mi16 that was fully rebuilt by a reputable company about 18 months ago or so and has only covered a few thousand miles since. All has been fine since the rebuild and it's a very sweet engine that makes good power, but its recently developed an odd oil pressure problem which I had a quick look at on Saturday for him.

 

Basically, the oil pressure is fine for the most part and with hot oil sits at about 1.5 bar idle and 4.5 bar @ 4000rpm, exactly as you'd expect. Give the engine a moderate amount of stick and everything is fine, and it will happily sit there at 100mph+ without worry. However, on full bore acceleration, the oil pressure slowly drops off and keeps slowly dropping until you let off the throttle and then it shoots back upto normal and will sit there happily.

 

The speed and amount of pressure it drops is worse the lower the gear you're in (and thus the faster its accelerating). Oil level is fine. Crank pulley bolt was slightly loose but I've tightened that. Pressure gauge and warning light tie-in so I'm assuming the senders and wiring are OK. No untoward noises or anything else obvious as the pressure starts to drop.

 

I'm guessing that it's either the oil pump drive is still slipping, or the oil pump is faulty despite having been replaced during the rebuild (I remember Maxi having similar issues on his engine that a new pump cured), but I'd value people's input if there's any other likely culprits that could cause these symptoms?

[smckeown 1]

As it seems revs based, doesn't that point to an oil pump finding it hard to keep up?

[Anthony 1,003]

As it seems revs based, doesn't that point to an oil pump finding it hard to keep up?

It will sit quite happily with full oil pressure at high revs on a constant throttle - just doesn't like it when you plant the throttle and accelerate hard (ie the engine is rapidly increasing rpm) in lower gears. So its more like it's related to the rate of acceleration/rpm change, rather than load or revs.

 

Thinking about it, the other test I should have done was to rev it hard when stationary in neutral and seen what it did, but didn't think of that at the time...

[Beastie 1]

I have just cured a vaguely similar problem on my 8v this weekend: I noticed on Saturday night that at sustained RPM above 4000 with any significant throttle opening, the oil pressure needle would steadily drop until it read almost nothing. As soon as I lifted off the oil pressure rose instantly to where I would want it to be. I found that I could drive on the overrun at any engine speed I liked and indicated oil pressure was fine. It was only under appreciable throttle openings that any problems were evident. At the time my mind was full with just about all the things which you have already thought of. It was just the fact that the guage returned to normal so instantaneously when I lifted off that made me think to try another sender unit first. It was the sender which was causing the fault and all is back to normal.

[smckeown 1]

I suppose that illustrates the golden rule i nthis area, if ever you think something isnt right with oil pressure, change the sender first

 

Same could be said about oil and water temp I suppose

[ashley peddle 3]

ant - had a text off him earlier saying its all fixed so i think you may have sorted it on saturday? could what you saw when you drive it be short term effects of tightening up the bolt?

[James_R 3]

Definately always check the sensors first, or peferably, plug in a mechanical one to test, as they dont' tend to go wrong at all.

 

My clio has been mucking about oil prssure wise like the above which I am blaming on the sensor, no.1 rule, never trust french electrics

[sam1312 0]

hi it it my mi thats the problem. its not fixed its self and gets worse the hotter the oil gets i think

[Anthony 1,003]

I'm not convinced it's a sensor issue though, unless there's two faults on the car (which admittedly isn't impossible). Prior to me tightening the crank pulley bolt it was dropping pressure much worse, and would drop so low that the oil pressure and STOP light would come on - remembering that the gauge and light are on different circuits and senders.

[Dino 3]

I had the same problem with mine a while back - it was the sender. Hope your mates is as simple as that dude.

[huxley309 0]

Isnt there some sort of woodruff key on the pump drive? or is it soley dependant on the friction of the crank pulley bolt which may have worked loose?

[JERAM 0]

had the same symptoms, presure dropping under hard accelerating when you come of throtle/load it was fine .....

 

found out it was stretching the rod bolts under load......

 

how did i find out....... when it through no3 rod through the block and earthed out starter live, then proceded to go on fire

[Anthony 1,003]

Isnt there some sort of woodruff key on the pump drive? or is it soley dependant on the friction of the crank pulley bolt which may have worked loose?

Nope, it's purely friction driven on the 1.9 Mi's, which is a bit of a rubbish design... I tightened the bolt as that was my first suspicion and it was slightly loose, but the issue persists.

[Beastie 1]

and would drop so low that the oil pressure and STOP light would come on - remembering that the gauge and light are on different circuits and senders.

 

Sorry - I didn't read that bit carefully enough so my previous post is less relevant. I agree that it doesn't sound like a sender problem in this case.

[hman205 1]

bad earth i have had a problem on my car that under hard accelleration both oil pressure and fuel gauge would go haywire and lots of lights would appear once i changed the altonaer and sorted out some untidy loom it cured it all bestbet is get a mechanical gauge on it and see if you can illimanate all electrical gremlins

 

could it be the oil pressure release spring is weal and just opening under sudden surge in presssure had it in a mini when i had some how snapped one.

[C_W 3 1 Cars]

Theorectically the oil pressure shouldn't be affected by acceleration since the oil pump is just pumping the same amount of oil round depending on revs, either constant revs or changes in revs.

 

Is the dipstick the right one?

 

Could it be an oil breather pressure problem of some kind?

[ada205 2]

I'm not convinced it's a sensor issue though, unless there's two faults on the car (which admittedly isn't impossible). Prior to me tightening the crank pulley bolt it was dropping pressure much worse, and would drop so low that the oil pressure and STOP light would come on - remembering that the gauge and light are on different circuits and senders.

I had a similar problem to this on a car i owned a few years back, and although i dont think it's the same fualt, mainly because of what Anthony says in the above quote, just thought I'd post in case others are looking into this thread who's problem has the same cause as mine.

 

The symptoms on mine were very similar, in that the oil presure would fall off when the car was driven hard, but no matter how low the gauge went, the oil light never came on. The cause was simply oil & crap on the connector to the sender unit. Pulled it off and gave the tag on the sender a wipe with a clean rag, and problem cured. This fault would return every so often as the connection got dirty again, and actually did it's little trick one time when a lad was test driving the car with a view to buying it of me! After some reasurance from me, he did just that and i waved good bye to the old girl!

[Anthony 1,003]

All fixed now.

 

Turns out that there were two seperate problems. Firstly there was the initial problems with the oil pressure randomly dropping out and doing so enough to bring the oil pressure and STOP lamps on, which turned out to be a crank pulley bolt that had worked its way loose.

 

Then there was a seperate problem that occured at pretty much the same time, where the oil pressure would drop off under hard acceleration, and as many of you correctly suggested, it was a faulty oil pressure sender. A couple of senders later, and the problem is sorted - no matter how many beans you give it, the pressure is rock solid.

 

So there you have it - just because you've got two problems with very similar symptoms, don't assume that its caused by the same things. Still, all fixed now, and Sam's a happy camper now

 

(oh, and driving his 205 Mi has definately got me wanting to pull my finger out and fix mine - soooo much fun )