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MG MGB Technical - Oil pressure problem
| Hi some sdvice needed when i turn my engine over(1800 mgb)it seams to take a long time to register oil pressure and when i start the engine up the guage moves up very slow. cold it reads 70LB and hot it reads 50LB this is a recon engine. I have changed the oil filter but it hase not made any difference. any advice would be appreciated Regards Garry |
| G HEATH |
| Garry, On the face of it your readings seem to be fine but just how long does the pressure take to rise when you start the engine, Perhaps more importantly how long does the pressure take to die away when you shut the engine down |
| Iain MacKintosh |
| Hi it takes about 20 seconds to rise to 70LB (engine running) and about 30 seconds to drop It is just the turning over that worries me as it nearly flattens the battery before it registers Regards Garry |
| G HEATH |
| Some MG's (my 1970) use an electric transmitter. Later models use a tube which directs the oil under pressure directly to the gauge. |
| Dan Robinson |
| Mine is a tube that goes to the guage (1971) Garry |
| G HEATH |
| Garry, Has it done this ever since the recon engine was put in? If so, it sounds like what happens when the wrong oil pump gasket is used. I know you changed filters, but do you know that the oil is not draining out of the filter on shut down? I am assuming here that you have the inverted filter. When I went through this with mine, I did the filter first, and then I added a mechanical oil pressure gauge to know that I was seeing the truth. Then I decided to pull the oil pan. I had indeed used the 3-main bearing oil pump gasket on a 5-main bearing car. Charley |
| C R Huff |
| I have no idea if it has been like this from being reconditioned as i bought the engine out of a braker with an invoice for £975.00 and showing approx 1000 miles I was just wondering if it could hav anything to do with the oil pressure relece valve Garry |
| G HEATH |
| Garry, it is the same with my engine (ST Stage 6). I fitted an uprated oil pump, filter adaptor and presure valve. New OEM filters or their substitutes with anti drain diaphragm did not cure the problem. For avoiding damage to the bearings, i spinn the engin with the starter at +/- 10 sec. intervals without choke and without any pedal presure for two to three times. Then the gauges comes in and it fires as soon as i pull the choke. I tried different wights of oil, but this did not cure anything. When the engine has been overhauled and the pump and filterbase have been modified to the recommandations of the ST booklet, it seems to be normal this way :-{. When presure is built up, it is no problem to let the engine sit over night as it will have presure immediately after a restart the other day. As i do not want to pull the engien again and change the pump and presure valve, i will have to live with it until a rebuilt is necessary. When i compare it to my V8, things are totally different. Even if the Car was set for weeks, it builds up presure immediately after it is fired. Cheers Ralph |
| Ralph |
| Garry, Then has it done this since you first got the engine, or did it start doing it later? And, how long have you had it? Do I understand correctly that you already have a mechanical gauge? Sorry, I don't know if the relief valve would do this or not. Charley |
| C R Huff |
| Garry, You say that "it is just the turning over that worries me as it nearly flattens the battery before it registers". Does the engine not start? If the engine is starting and it's taking 30 seconds then there is a big problem. But you say that it takes 20 seconds to drop. Have you tried a different gauge? Or maybe a blockage in the gauge pipe? Neil |
| Neil |
| Garry, Sounds like the oil pressure is ok once started but starting is your problem. Are you saying that once started the oil pressure goes to 70psi? That is good. And runs at 50psi? That is good. The slowness of the pressure rise after starting may be due to cold starts and the heavy oil you are using. If it is the spec 20W50 then it is pretty thick when cold and may be slow to register. How does it rise when the engine is restarted after having run for a while, is hot and shut down? |
| Barc Cunningham |
| It would have to be treacle to take 30 seconds. And as for an engine keeping pressure overnight, well where is it being stored? The oil system is full of holes, so the pressure should drop immediately. The ST mods only smooth the passages into the pump - ie to reduce input drag and hence improve flow and reduce the chance of cavitation. The first thing to do is check the pressure with another gauge. Neil |
| Neil |
| The engine starts first throw but i would like oil to be circulating befor it starts.Yes it has done this since i have had it wich is only 7 days. after the intial start if i leave it for 10 minutes it starts with good oil pressure but leave it over night that is when i get a problem and the oil guage was ok on the last engine even with a rumbling crank Garry |
| G HEATH |
| Garry, Is your oil filter the inverted type? If so, does your filter bracket have the stand pipe in the middle of it that keeps the oil from running down the center hole on shutdown. If it does, then I would make sure that your filter anti-return flap is working. You can tell this by removing it after it stands overnight, and see if it is still mostly full. I think it won't be absolutely full because it will drain to the level of the stand pipe in the center of the bracket. Also, some filters are longer than others. I suspect that a longer filter will loose more oil when standing because it stands taller in relation to the center pipe in the bracket. If none of this reveals a problem, then decide if you are absolutely sure your gauge is telling you the truth. If you get past all of this without a solution, I would be inclined to pull the oil pan and see if it has the wrong oil pump gasket. Yours is doing exactly what mine did except that mine took about 25 or 30 seconds to make pressure, and it made about 60 psi warm. You can pull the pan without pulling the engine. Charley |
| C R Huff |
| Let me check that I understand this Gary. You turn the engine over, without starting the car, until oil pressure shows. This takes about 20 seconds. You then apply throttle and/or choke and the car starts instantly. The oil pressure is about 70 then. When you stop the engine it takes around 30 sec before the oils pressure gauge drops back to zero. If you start the car after it has been used then the oil pressure comes up far quicker. Is this all correct? If so then you have an engine in excellent condition and an oil filter without a good anti return valve. Most, if not all, oil filters in the UK don't have the return valve. What you will find is that the initial cranking is filling the oil filter. If you just start the car then it will take 2 or 3 seconds for the oil pressure to shoot up to 70. The fact that you get a good reading of 70, drop to 50 psi when hot, suggests the engine bearings are good. Also the drop off time suggests very tight bearings. A worn engine will see it's pressure drop almost instantly. If you want to solve this you have two options. Find an oil filter that will hold it's oil over night (it probably still won't hold over a week or so), or switch the car to a hanging filter. The MGOC and Moss sell hanging adapters so that you can have a hanging screw on filter, but I am not sure they are suitable for all engines. Cheers Iain 67 BGT |
| I D Cameron |
| Iain, I thought Garry said that it takes 20 seconds to make pressure after he starts it: "Hi it takes about 20 seconds to rise to 70LB (engine running) " Charley |
| C R Huff |
| UK never used the electric gauge, and the US used the mechanical before 67 as well. 20 seconds to rise and 30 secs to drop (not the other way round) seems very slow for a 4-cylinder to me. Mine takes about 10 secs, but only after standing a couple of weeks or more, and that depends on what type of filter I have fitted. Mine drops *very* much quicker than 30 secs. The V8 with its very much longer oil pipe run takes about those times when cold, much quicker when hot. I'd suspect a pinched gauge pipe, or a very slow-acting gauge for some reason. So the first check should be a different gauge on the end of the existing pipe, then a different pipe from the block as well. You say it nearly flattens the battery before it registers, are you killing the ignition until you get pressure? I can't really see the point in that myself, there is little difference to the bearings cranking with the plugs in (unless the throttle is wide open which would flood it) and running at a gentle fast-idle once started. Roaring off before you have oil pressure is a different matter. The other thing is to listen carefully to the engine between starting and seeing oil pressure. You may well be able to hear the difference in the bearing between oil pressure and no oil pressure, and I don't mean knocking but just a different quality of sound. If you get that sound change noticeably before the gauge starts rising then the problem *is* in the gauge or its pipe. |
| Paul Hunt |
| Ian, Why does a long 'drop off time' for the oil pressure suggest tight bearings? Neil |
| Neil |
| Update probelem solved chainged flexi pipe from engine to bulkhead now oil pressure straight there. Thankyou for all your helpfull advice Regards Garry |
| G HEATH |
| Garry, Aren't you a lucky dog! Neil, If the bearings are loose, all the built up oil pressure dissipates quickly through the slack between the crank and the bearings on shut down. If they are tight, the built up oil pressure finds it harder to get out, and so delays the drop of oil pressure. The lazy drop is quite evident and satisfying on my fresh engine. Charley |
| C R Huff |
| Amazing, I had noticed the same behaviour on mine, and always crank it with switched off ignition through a modified circuitry to buil up pressure. I hardly can imagine the flexi pipe to produce such a result. Thanks for the feed back. Rgds. |
| Renou |
| "I hardly can imagine the flexi pipe to produce such a result" Oil has to flow through the pipe to fill the Bourdon tube and expand it to move the pointer. You wouldn't expect the gauge to move at all if the pipe were completely blocked, and it follows that a *restriction* will *slow* the movement of the gauge, both up and down. |
| Paul Hunt |
| Paul, I noticed this latency at engine cranck stage only, not a shutdown. I asked many advices for an explanation, but none came. I supposed my gauge a bit lazzy at build up step: it's a 63 one after all. Rgds. |
| Renou |
| Oil has to flow back out of the gauge when you switch off, so I'd expect the same resistance to flow and hence the same sort of delay. However if you only start from cold and only switch off when hot then it *will* drop faster than it rises as the warmer oil will flow through the restriction more easily. If you start and stop at the same temperatures then it *should* be much the same. That it how it is on my V8 anyway. |
| Paul Hunt |
| Difference is with rising pressure the pump is trying to pump against the restriction with constant pressure. With falling pressure, ie at switch off, the pressure in the gauge redueces as the oil escapes through the restriction thereby causing the pressure pushing against the restriction thereby reducing the flow etc etc So it makes perfect sense for the problem to be the gauge pipe, and good sense for there to be more of a problem at shut down than after start up. N |
| Neil |
| Hello. I would like to comment on the oil in the pipe theory. I think that the pipe is full of nothing but air unless there is a leak at the gauge that allows the oil to travel up the pipe. The oil simply compresses the air in the pipe in the same way as trapped air gets compressed in the brake lines. I can't see why oil would fill the pipe in normal operation. Tony |
| Tony Oliver |
| Had the same thing. Chased the problem; scratched my head for weeks. Turned out the oil pump rotor and body was worn and scored. Ordered a new pump. The new pump that arrived was a reproduction and the rotor end gaps were out of spec and there were bits of casting flash inside the ports. A cleanup, and an hour or so with some fine emory paper tightened the excess gaps. Now I get 70 psi at startup and while running. At idle in really hot weather, I get about 65 psi. |
| Daniel Wong |
| Interesting, I had to press the 74.5 MGB into everyday service as my daughter's car ceased to run and she takes my daily. The weather just started to get cold here, low 30's F and I noticed a slow rise in pressure at first start, never had this happen in the warmer weather, does get up to 75 a short time after start but definitely longer than the warmer weather. Drops to 50 after she warms. I'll have to time it next time. |
| Luis |
| Tony - the MGOC actually recommended *bleeding* the pipe to try and reduce the standard guage rise time on the V8. For the reasons I mentioned I felt this would actually make things worse as the V8 pipe is a very small bore for a very long distance, especially when it comes off the pump like mine does. I did a test by bleeding it to make it full of oil, and also the reverse i.e. blowing down the tube to *empty* it of oil, but could see no difference in gauge rise and fall time. But that was with the normal delay on these cars, Garry had what I would regard as very excessive delay on his 4-cylinder. He resolved it by replacing the flex pipe which comes off the block, so the restriction was in that and that is very close to the block and the galleries that *are* full of oil. Oil is bound to move up this pipe, for the very good reason that any air in the pipes and gauge will be *compressed* to the oil pressure of 50-70psi, as well as because of the Bourden tube expanding, but to what extent oil moves up an empty pipe on my roadster or V8 I don't know. I do have a spare gauge with a clear plastic pipe. I've not connected it up to either of my MGBs but I did to a Celica when I was investigating low oil pressure readings on the dash gauge. The block source is left front on that, and I had pipe to spare when I mounted the gauge in the cabin on the right-hand side i.e. a much longer distance than the 4-cylinder flex and metal pipes. In use oil filled the first third or so of that pipe, i.e. much further than the length of the 4-cylinder flex pipe. |
| Paul Hunt |
| Assuming that the oil is effectively incompressible, then an increase in pressure from 0psi-gauge to 70psi-gauge (85psi absolute) will cause the air in the tube/gauge to compress about six fold - so oil is *bound* to flow up the pipe... and any in any case, Garry has said that changing the pipe has fixed the problem! As an engineer, I would always consider whether the results (in this case the pressures shown on the gauge) are correct before looking for a cause. It would be really, really annoying to dismantle an engine only to find that the problem was on the dashboard. N |
| Neil |
| Garry, good to hear the problem is gone. I don't know whether you read about the problems with the aftermarked flex tube for the oil resure line to the instrument, posted here in the past. There have been some failiures within short time due to poor quality pipes used on the spare part. Mine lasted for +/- 2000 miles and only my non stock presure warning light saved me from wracking my engine. A frind had the same problem when the pipe deteorates within a few K Km and needed his krank to be done. The one i use now has been pressed to the old fittings by a local hydraulics service. They were realy amazed upon the China quality hose used underneath the nice blairing! Just check and think about it! Cheers Ralph |
| Ralph |
| Point taken Paul. Thanks Tony |
| Tony Oliver |
| Ralph, or anyone who knows, To add a low oil pressure warning light to my MGs, am I correct that I need to plumb in a separate electric oil pressure sending unit? My 77 is mechanical and my 68 is electric. I am guessing the electric sender on the 68 won't work for a light, so that the upgrade would be the same on both cars. If I am right so far, can someone tell me what sending unit to buy? And, on the oil/air in the sending unit tube, I had a mechanical gauge with a clear pipe on my 68 for a while, and some segments of the pipe had oil in it and some segments had air. Thanks, Charley |
| C R Huff |
| Charley, on my roadster (1974) i used the fitting for the switch of the anti run on valve for a presure switch, it is connected to the check switch of the brake faliure light on the dash and has proved to be a valuable update. The brake faliure valve is set out of funktion with it's internal switch removed an pluged (silly extra due to US legeislation authorities in the early 1970's). Brakes work or they have a (never experienced within more than 30 MGB years) malfunction. With this cars with an aditional red warning light - what does this help for? It can be used for an oil presure warning light with much more benefits, i learned. If you run an electric presure gauge, there are adaptors availabel for the interconneting pipe fitting that have a thread for a presure switch too. just have a look at items offed at speed shops or ask the VDO distributers. They fabricate and stock different versions that can be used. On my V8 i had a hydraulic shop drilled the offset filter base and had cut in a thred for a stock presure switch as used on other Leyland cars. It is also wired to the brake presure failure switch, as on the roadster. Hope this helps Ralph |
| Ralph |
| Thanks Ralph, The set up on your 74 might be perfect for my 77 since the brake failure switch is broken at the master and I wasn't willing to spend money for a light to tell me that the brake pedal just went half way to the floor. Also, the anti run on system has been disabled. Do you know what the oil pressure drops to when it activates the anti run on valve? Charley |
| C R Huff |
| Charley - you can't use the anti-runon oil pressure switch - not unless you want the light to be on all the time or use it with a relay, as it operates the other way round to an oil pressure switch. The anti-runon oil switch is normally open and *closes* as soon as any oil pressure develops, and sends an earth/ground to one side of the valve. But it is only when the ignition is turned off that the ignition switch sends 12v to the valve, which operates. If the ignition hasn't stopped the engine then the valve does, and when oil pressure has died away the anti-runon valve releases again. The next time the engine is started turning the ignition switch on removes 12v from that side of the valve *before* oil pressure can develop to send an earth to the valve. You want an oil presure switche that is normally closed and *opens* with oil pressure, The switching pressures vary, but anything around 2-5psi should be fine. Make sure the thread fits the adapter as they can vary as well. But many years ago I watched a Scimitar GTE scream off the line at a hill-climb, to leave me wondering what that big orange light (a classic Mini front indicator light unit) on top of his dashboard signified, then noticed the huge and spreading pool of oil where he had been standing seconds before. I did several hundred yards at full-chat before he noticed. He fitted a new oil filter, and proceeded to win his class. The moral of the story? You can miss a light as well as a gauge. I wired my oil and ignition warning lights via an exclusive-OR gate to a buzzer, which meant that if either came on while I was driving the buzzer would sound, but it wouldn't when starting the engine when both were lit. |
| Paul Hunt |
| Thanks Paul, I guess Ralph meant he used the port for the anti-run on switch rather than the switch itself. Yes, a buzzer would be a good idea. I used to have them on trucks that I drove. Actually those were a buzzer and shut down system for low oil pressure or high oil pressure. They did have a manual override button that you could hold if safety was more important than the engine. Charley |
| C R Huff |
| Early 40hp. VW beetles had an oil pressure switch that turned the light on at about 7-8 lbs oil p[ressure. They also had and adjustment screw right down the center post where the wire was attached. It could be adjusted quite a few pounds either way.If my memory is correct it woud be approximately the same size threaded portion as a TD brake light switch. Sandy |
| conrad sanders |
| Garry, i used the oil presure warning switch from a BMC Mini, but there are many others that wil fit. The sitch of the anti run on valve would make additional wiring necessary, as Paul allreday stated. There are also ready built buzzers available for different marine engines. I saw it as a stock panel outfit on Volvo marine diesels. Ralph |
| Ralph |
| When I said, "...and shut down system for low oil pressure or high oil pressure ..." I meant low oil pressure or high temperature. Charley |
| C R Huff |
This thread was discussed between 17/11/2008 and 24/11/2008
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