Knocking on start up

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gary_nz
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Knocking on start up

#1 Post by gary_nz » Thu Jul 16, 2020 11:51 pm

Hi,
I have a '76 2500TC which is knocking on start up. I've had the vehicle for around 18 months and it has always knocked a little but it now seems to be knocking for longer now.
The knocking now lasts for around 30 seconds from starting. I leave the car at idle on choke and the oil light is lit during this period. The knocking stops and the oil light goes off simultaneously.

The oil light has never come on while driving, even at idle and the engine sounds fine when oil pressure has built up.
I changed the oil and filter when I first bought the vehicle but the supplier could not find the actual filter (spin on) so we found one that fit the car and ran with that. It's a short filter so may not have non return valve. I've just ordered an oil pressure gauge and I'm going to try to track down a filter with a non return valve and also better oil with zinc additive.

I guess my question is, should I be concerned about an imminent bearing failure or not?
I know I could crank the car with the choke in until pressure comes up but that would mean a lot of cranking.

Thanks in advance.

tony
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Re: Knocking on start up

#2 Post by tony » Fri Jul 17, 2020 3:47 am

Not really a good trick to run it like this.you don't necessarily need a non return valve in the filter, the spin on filter is fitted vertically so drain back shouldn't be a problem.
The sooner you can get a pressure gauge fitted the better so you can see whats happening . Use a good quality 30/50 grade oil..
It's probably worn big end bearings but see if the new oil helps.Depending on what the oil gauge shows, hot should be around 50psi, you could also check the pressure relief valve and spring. but afraid my money would be on the bearing shells.
I wouldn't be too worried unless it gets worse. A lot of these motors did this before fitting spin on filters.
Have fun.
Tony.
1976 2500 TC. converted to S specs.

Lots of bits

1999 BMW Z3.
2006 BMW 325ti.
Hopefully not needing too many bits.
1998 Rover 600



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Phil T
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Re: Knocking on start up

#3 Post by Phil T » Fri Jul 17, 2020 8:19 am

You don't say if you have checked the oil level recently, if it is low it will take longer to build the pressure up.

Does sound like big ends, similar to mine........ :oops:

Mike Stevens
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Re: Knocking on start up

#4 Post by Mike Stevens » Fri Jul 17, 2020 11:40 am

I'm sure the knocking is the big ends, but whether it is a problem or not is a different matter.

The main issue you have is the slow build-up of the oil pressure. I had a similar issue with my PI, especially after changing the engine oil. It has a spin-on filter with it hanging down. In the end I fitted a new oil pump and in doing so found that the engine had been rebuilt (before my time!) using lots of white silicon. This had got into the pump inlet (which was an early Mk2 type) and semi-blocked the inlet gauze. The new pump and a good clean sorted this out.

As long as your engine is quiet when the pressure is up I don't think you have an issue. Oil pressure (according to the book) should be 40-60 PSI at 2000 RPM, engine at running temperature.

Here is a picture of the old oil pump. The wire like black stuff to the left is the silicon!
DSCF9743a.JPG
Cheers,
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Clifford Pope
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Re: Knocking on start up

#5 Post by Clifford Pope » Fri Jul 17, 2020 2:28 pm

Does the engine fire immediately? Is that because the petrol doesn't run back, or because you prime the fuel pump?
If the oil pressure is a bit sluggish to build up I think it's worth cranking the engine a bit to get the fuel through, which ensures that it gets the oil ready too and avoids running with low pressure.

Spin-on oil filter conversions need checking occasionally to see the O-rings haven't deteriorated. If the inner O-ring is leaking you can't tell but the oil bypasses the system.

Worth checking the oil pump tolerances. If the end float is too large you can grind down the face of the pump.

Is there an oil pressure feed pipe for the valve gear? Not now recommended as much as formerly, as it tends to rob the lower engine of oil by diverting too much up top.

gary_nz
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Re: Knocking on start up

#6 Post by gary_nz » Mon Jul 20, 2020 9:39 pm

Thank you everyone for your informative replies.

I had some time over the weekend to fit an oil pressure gauge and filter and oil change and now have some results.

When I first purchased the car, about 18 months ago, I changed the oil and filter. My supplier couldn't supply the Z87A that I took off so he found a substitute (Fram PH966B) and I used Shell Helix HX3 20W 50 oil. Everything seemed okay until the knocking on startup period seemed to be extending (sometimes up to 40sec), hence my questions here.

Recently, before changing the oil and filter, I fitted the oil pressure gauge (I saw what I was expecting, very low pressure at startup for a while until the pressure came up to 20psi at idle. What I didn't expect was the pressure remaining at 20psi when at speed. In fact when it warmed up and I was cruising the pressure dropped even more (15-10psi) and remained that way for the duration of the 15km trip. As you could image I was a little concerned.

I managed to track down a Z87A filter and Penrite HPR30 20W 60 high zinc and after changing oil and filter the pressure came up to 60psi and remained fairly consistent. Not sure if the oil or the filter, or both made the difference but I'm happier with that of course.
I'm yet to do a test from cold to see how long the pressure takes to build so I'll let you know how it goes.

Thanks again.

tony
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Re: Knocking on start up

#7 Post by tony » Tue Jul 21, 2020 2:44 am

FWIW I have always used a Z103 filter which is fairly long, with no apparent problems. I also use Penrite 20/60 oil.
Tony.
1976 2500 TC. converted to S specs.

Lots of bits

1999 BMW Z3.
2006 BMW 325ti.
Hopefully not needing too many bits.
1998 Rover 600



.

gary_nz
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Re: Knocking on start up

#8 Post by gary_nz » Tue Jul 21, 2020 11:54 pm

Just updating...
I started the engine from cold and after a little cranking it fired up and the pressure came up to 60psi almost straight away so that's good news.
It's interesting that a different oil filter and new oil can change this situation so drastically as It's not like the oil was that old or low.
Anyway I hope this information helps someone else.

torque2me
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Re: Knocking on start up

#9 Post by torque2me » Tue Jul 28, 2020 11:31 am

gary_nz wrote:
Mon Jul 20, 2020 9:39 pm
cut

I managed to track down a Z87A filter and Penrite HPR30 20W 60 high zinc and after changing oil and filter the pressure came up to 60psi and remained fairly consistent. Not sure if the oil or the filter, or both made the difference but I'm happier with that of course.
I'm yet to do a test from cold to see how long the pressure takes to build so I'll let you know how it goes.

Thanks again.

Do you know what amount, in ppm (parts per million), Penrite put in their 20w60 (which I used a few years ago after being offered 1/2 doz. containers of it a very nice price?

Yes, filter manufacturer (not make) can make a difference. I think we may have a forum member who used to work for Perkins and did some R&D work with filters. Hope he chips in.

regards,

Kev

gary_nz
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Re: Knocking on start up

#10 Post by gary_nz » Mon Aug 03, 2020 11:50 pm

Another update...

I was chatting with a mechanic about this and he asked me if I'd had a petrol leak recently and my answer was yes. His next question was "Did you smell fuel when changing the oil?". I think you can see where this is going but I can't say I noticed it.
I won't go into detail about that but suffice it to say there was a leak from the fuel bowl attached to the carb (out of the breather port on top).
If enough fuel leaked into the engine via the carb, it would dilute the oil enough to lower pressure.
I think this scenario is quite likely and I still have the old oil so I'll check it for fuel smell and also viscosity.

I've driven several hundred km now and pressure is holding well.

Thanks again to all who contribute to these questions.

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