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MG MGB Technical - POW!!! SPUTTERSPUTTERSPUTTER!

First a little history on the problem. For the past few months I've been plagued with a sputtering problem at high speed and under load. Travelling over 60 or so MPH, I would get the occasional loss of power and sputtering as if running on three cylinders. If I backed off and waited a few seconds the engine would normally smooth back out to normal. I thought it was a carburation problem but couldn't get rid of it with adjustments.

Tonight I went for a nice spin starting with freeway driving. Sure enough the sputtering was back and bad as it had ever been. Things went fine once I got off the freeway. Next I had to show a monsterous Lincoln SUV what a real car could do when he thought he could outmanuever me in the canyons. I felt bad because he appeared to nearly roll over a few times before I finally decided to ditch him for good. No engine problems of any kind until I got back on the freeway.

After a mile on the freeway travelling at 70-75 MPH, the sputtering returned but this time there was a "pop" and a cloud of white smoke from the rear of the car. Oil and temp were fine so I drove it carefully to the next exit and drove slowly to diagnose the problem.

My first thought was freeze plug due to the pop, the cloud of smoke and the sputtering (maybe a wet distro). I made it to a gas station and popped the hood. My hood insulation had been steamed off and there were specks of coolant here and there but the freeze plugs were all in place. Temp was pegged at max after I stopped. I messed around, finding nothing, so I fired up the car. It ran perfectly! I decided to try and make it home which was only a mile or two away. I made it a couple blocks before the engine got rattley and started backfiring so I shut it down. It did this a few more times to get it home which wasw perhaps a stupid thing to do but the car made it without overheating or any problems outside horrendous loss of power and smoothness, and a couple nasty knocks.

My only guess now is a cracked head? I would love some advise about where to start looking. I don't want to get more involved in this than necessary since I'm not much of an engine rebuilder. I'm guessing I need to pull the head regardless. The kicker is, there is only 3,800 miles on this engine! Sorry so long. If you read the whole thing you get a cookie! Thanks all.
Steve Simmons

Steve - Your problem sounds so familiar that it hurts. I am having the same type of problem with my MGB (66). The only way I found what the problem is, was to do a leak down test on each cylinder. After having done a compression test and seeing things all look ok, I did a leak down test and found a leak between cylinders 2 & 3. Seems that the head and/or top of the block have a slight bow in them, allowing combustion pressure to go between the two cylinders and, in my case, a bit into the cooling passages. Engine is at the machinist now for an attitude adjustment. Good luck - Dave PS - do I get the cookie?
David DuBois

Steve, Do you think the white smoke was from the exhaust or was it from engine bay and under the car? The fact you the under hood/bonnet insulation was steamed off and coolant droplets were in the engine bay would indicate the coolant came out from somewhere in the engine compartment. Look carefully at your radiator and heater hoses for signs of a split. If all hose are good check the coolant level and check the engine oil for water content. Run a compression check. If compression isn't correct do as David suggested and run a leak down test. After running these checks you should have some indication of how serious your problem is. Good luck, Clifton


Clifton Gordon

The "smoke" came from the engine bay, not the exhaust. Steam was pouring out of the front of the car when I opened the hood. It didn't last long, and the car fired right up again. Strange.

I have a compression gauge and will test today. But what is a "leak down test"?

David, I will email you a cookie as soon as my new matter digitizer arrives. Hopefully you have one on your end to convert it back into a cookie. :)
Steve Simmons

Steve, sure sounds like a head gasket leak.
The sudden rise in temperature indicates combustion gases getting into the coolant.

The droplets of coolant inside the engine bay
could be due to an external leak at the head gasket.
Check the block/head interface carefully and a leak
might be evident.

You can buy a "block tester" at NAPA or other
autoparts store -- this is a tube that is filled
with blue fluid. Put the tube inside the radiator
cap opening or expansion tank opening (77 or later).
If the fluid turns yellow, you have a bad head
gasket or cracked head.

A conventional compression test showing poor readings
on two adjacent cylinder would also help confirm
a gasket leak.

You say the engine was rebuilt 3800 miles ago?
Was the head ever retorqued (should have been
at least at 1000 miles.)? Maybe the rebuilder
has an implied warranty and will make good?
Ronald

The head was retorqued not long ago. The rebuild was done years ago so I doubt the guy who did it will warranty anything. The car sat for a few years without being driven.
Steve Simmons

Steve, do a leak down test regardless of what the out come of the compression test is. As I stated in my earlier post, the compression test on my engine did not show anything amiss (150, 145, 135, 150), just two cylinders being slightly low. The leak down test showed 4 % leakage for #1, 35% for #2, 40% for #3, and 8% for #4. Checking at the oil fill, carburetors and exhaust, indicated that air was escaping from #s 2 & 3 through the carburetors (this even though the valves had been redone recently and a vacuum test of the head showed everything good). After checking all the cylinders for leakage, I popped the radiator cap and got a release of pressure, indicating that some of the leakage had gone into the cooling system. When I pulled the head, it could be seen where the cylinders were leaking from one to the other past the head gasket.

A leak down test is the pressurization of each cylinder in turn while it it at top dead center with the valves closed. The cylinder is pressurized through a calibrated orfice with a gauge on either side of the orfice. The up stream gauge reads input pressure to the orfice and the down stream gague reads the pressure being felt in the cylinder. The difference between the readings is the result of leakage from the cylinder and read as a percentage. By listening at the oil fill, carburetors, and exhaust, you can get an idea of where the air is leaking from. Leak down testers can be purchased from Eastwood or you can make your own, eithe way, it is a good diagnostic tool to have.

Good luck,- Dave
David DuBois

Thanks. I'll look into it. Anyone know where to buy one without ordering? I'd love to dig into this over the weekend.
Steve Simmons

Steve, A leak down test injects a calibrated air supply through a tester with either dual gauges or in my case a single gauge. Any leakage through valves or around the rings will be read in a percentage or a good/bad scale. See an example on my Yahoo Photos. Click on the, "cylinder leak down tester", album.
http://photos.yahoo.com/flash1929
To run the test the cylinder being checked is turned to TDC on the compression stroke/both valves closed.
Hook up the tester and if it shows a leak you can usually locate the leak source by the sound of the leaking air. Exhaust sound would indicate a leak in the exhaust valve, Air cleaner sound would be intake valve and valve cover oil filler leak would be piston ring blow by.

After thinking more about what you said happened I think your engine may pass the leak down test, but if your compression is down in one or two cylinders it may be worthwhile to run one. Since your engine ran well after the smoke came out I would think your problem may not be a head gasket. Of greater concern is that it was driven home and you heard engine rattles, hopefully the rattle was pinging/pinking due to overheating. I assume coolant is low or the system is empty.

I have seen the split radiator or heater hose happen two times and both of them made a sudden noise. First time was many years ago in a Volvo PV 544, top radiator hose was split and spraying coolant on the engine fan. The next was with a friend in a Dodge PU. We pased a truck my friend got up to 90+ mph and he said, " what's all that smoke behind me, I looked back and there was so much smoke I couldn't see the truck we had blown by, I suspect the truck driver is still laughing. A heater hose had split and was spraying coolant on the exhaust manifold.

Hope your problems aren't too serious. Clifton

Clifton Gordon

Dave, I didn't intend to duplicate what you wrote but being a slow two figer hunt and peck typer I was busy trying to type and didn't realize you had already posted. Regards, Clifton
Clifton Gordon

After you get all that fun stuff done and fixed, make sure you have a good fuel pump pressure and clean filter. Also check your float levels. Sounds to me like the original symptoms were due to lean mixture during steady running. Canyon running had enough time off the throttle to allow the gas to keep up with the requirements. The steady running on the freeway didn't have any time off the throttle.

Good luck,

Edd Weninger

That brings up a question that's been in my mind for a while. How do you check fuel pressure?
Steve Simmons

With an in-line fuel pressure gauge, typically on a tee fitting which you insert, say, between the fuel line and the rear carb float bowl. The pressure is quite low to start with so you want a gauge with calibrations capable of registering it.
I had similar symptoms that were caused by a blocked fuel filter.
Andrew Blackley

Steve - The pressure that you should see with a typical AUF 300 or AZX 1300 series SU fuel pump should be on the order of 1.5 to 3 psi. The other thing you need to check is flow rate. Get a 2 pint measuring cup and direct the open end of the fuel line into is and turn the pump on for 1 minute. You should get a pint in about 30 seconds.
Good luck - Dave
David DuBois

Thanks to all for the info. I finally got to play with the car. What I found is that I have two seperate problems. One is the loss of power at highway speeds. The other is the reason there is no coolant in my engine. Ready for this?

I blew the rear freeze plug between the engine and transmission. The one you can't get to. Not on the side of the engine, but the one facing the cockpit. This is the second freeze plug to blow on this engine and another (rearmost on side of engine) is leaking. I want to kill the guy who put these things in.

I'm wondering if there is a relation to these two problems or if it's just coincidence that it picked just that time to blow out - just as the sputtering started. It may be coincidence as I was pushing the car a bit hard which could cause both symptoms.

Now, if I may ask opinions that I dread hearing... can this plug be replaced without pulling the engine? Sure doesn't look that way.
Steve Simmons

Steve, It has been replaced with the engine in place, I recall it being discussed in BBS threads in the past, however I don't recall the details. The information you need should be in the archives, or maybe someone that has done it will respond. As I write I think I remember Chris Betson may have made a suggestion on how to install a rear plug with the engine in place. Good luck, Clifton
Clifton Gordon

I have not tried to install one one the very back of the engine, but perhaps a Dorman "Quickseal" would work. It may be able to fit between the engine plate and the block. It is a copper plug that expands as you tighten it. It could be easier than the stock plug, especially at that position. I have one intalled at the rear most freeze plug on the side of the block and it works very well.
I got mine at a local independent parts store that carries Federated and other brands.
Andrew Blackley

Compression test shows 150psi on cyl 1, 125 on 2 and 140 on 3 and 4. It takes about 3 seconds to get to full pressure. Does this sound normal?
Steve Simmons

I have had an MGB develope a miss when hot and it was very hard to find. Finally I found a very small hairline crack that would open up enough to leak coolant onto the distributer. It was located in a vertical plane just in front of # 3 spark plug.At times it woul;d kil the engine completely. Let it set for a while and it would start and run perfectly until you ran hard and then would re-open and spray the cap,
Sandy Sanders
conrad sanders

Cylinder 2 compression is definitely way down.
This could be due to a burnt valve, but I'm
still betting on a head gasket leak or
cracked head.

Furthermore, I wonder if the
leaking combustion gases into the coolant are
causing the freeze plugs to blow out in the first
place.

But I could be entirely wrong.
Ronald

I don't see anything resembling coolant in the oil in the rocker area. I will probably change the oil to be sure. It would be very obvious if there were coolant in the oil, right? Even a little would seperate out I'd imagine.

I can't find a leak-down test tool anywhere local so I guess this project will have to wait until I can order one.
Steve Simmons

It is possible that fuel starvation initiated the problem. Running lean will increase temperatures quickly, so don't forget that after you get done with the freeze plugs and head work. Regardless, the freeze plugs shouldn't have blown, the radiator cap should have. Replace them all. The Dorman "Quickseal" or equivalant may be found at boating supply shops. I used to carry them in my go-fast boat and have replaced them "on-the-water" and go terribly seasick in the process. You won't need them in all places but might be worth a try for engine-in-the-car.

I think the leak down test will tell you what you already know,.. you have to pull the head.

Good luck
Edd Weninger

Does anyone have the plans to build a leak-down tester? I can't find one anywhere.

Also I can't find the Quickseal plugs. Everyone has to order them. I'll try boating shops. THanks for the tip!
Steve Simmons

Steve - Didn't the article on the leak down tester that I sent to you have the drawing with it? Let me know if it was not there and I will resend it. There was a drawing, bill of materials and detailed instructions of how to build the tester (since the article was a Word document with a picture embeded, you might have to click on the article and open it seperately). Good luck - Dave
David DuBois

Boy do I feel silly. I never noticed the instructions in the word document. Obviously I haven't read everything you sent yet (there's a lot there!) but still that's no excuse. THanks again for the info!
Steve Simmons

This thread was discussed between 23/05/2003 and 24/05/2003

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