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MG TD TF 1500 - vapor lock diagnosis
|1954 mgtf: Jet hot applied to exhaust manifold, put 5 cores in radiator, installed mgb fan, installed electric fan on radiator, installed b&g heat shield, wrapped entire fuel line, installed water pump with extra fins and still stalls after 15 minutes of running. Float in carb bowl is at 7/16in. Front 3 exhaust manifolds are 350-380F while the rear exhaust manifold is over 500F. Is this my problem? What can cause the one manifold to be hotter than the rest?|
|hello Mr C.Eastman. this is my meaning on this problem. Thefans do not cool down the manifolds. They just cools down the water in the radiator and the hot air from the radiator is blown back past past the manifolds the result being higher temp on |
|t g sorensen|
|The water in the radiator doesn't get hot enough for the radiator fan thermostat to turn on the radiator fan: it vapor locks before that.|
|You do not have vapor lock. (still stalls after 15 minutes of running). Vapor lock only happens after a hot soak, I.E. engine shut down for a few minutes to a bout a half hour, depends on ambient temperature.At this point engine starts but runs poorly for a few minutes.You may have an electrical issue.|
|Len is right. When it stalls, remove the float chamber lids and see if they are full of fuel or dry. If full, problem is ignition. If dry, a fuel problem (pump failure, blocked gas cap or clogged line, etc. George|
|Is there a chance your plug wires are crossed? (firing order wrong) Check too see you are getting same spark to all 4 plugs. Or could be you have a leak on #4 so it is running hotter. This should be easy to find with a bit of spray. I would also take a close look at intake manifold. Perhaps the equalizer tube is way off. (Check the bolt on underside of the manifold.) I also have not heard many good things about heat shields...the one I tried was useless.|
David 55 TF1500 #7427
|Have you improved your fuel cap seal to the point where it is airtight? (It should not be).|
When it stalls out, what color are the plugs ???? This would give you an indication of the fuel mixture in each cylindar...
|I had a similar problem of my TF shutting down after about 10 minutes run time - just long enough to get to the far side of the track on the first lap! I went through the fuel system 4 times and still no fix - just could not figure it out. Lost an entire race weekend. Turns out the distributer shaft/housing was so worn that the slightest expansion from heat would cause it to loose it's ground! I ran a ground lead from the housing to the block and was up and running until I was able to get it rebuilt. So try checking your ignition immediately after it shuts down on you.|
|What is disturbing here, (and I don't recall ever seeing this one on this BBS before) is the vast temp change on #4 when compared to the other 3. What could cause just one cylinder to run so hot and/or lean? Wouldn't a "leak" of some kind be about the only thing that could cause this?|
Perhaps in the head between intake and exhaust?
PS : Mr. Eastman,
These guys are good they WILL figure it out!
Hang in there.
| Not the same MG but I had a 77 midget that did the same as soon as the temp got a little warm outside. I finally solved the problem by installing an electric pump back near the tank.|
I think much of the probelm was due to the gas line being so small.
But a small gas line would not raise the temp at the rear exhaust manifold!
|How about something with the valves???|
|I had a very similar problem when I first got my TD about 10 years ago. It would run for about 15 minutes or less, then it would stop and nothing would get it going! I would push it back to my garage, and the next day it would start right up and the cycle would repeat itself. Before I picked the car up, the previous owner had a very popular MG guy rebuild the carbs. When he attempted to get the car home, he got stuck on his way! Next the guy charged him to replace the fuel pump. I checked out the fuel system and could find nothing wrong. I then replaced the points, plugs, cap, candenser, wires, etc. Nothing helped! It turned out to be the coil. Never had a problem since. Still running the same points, etc after about 11000 miles.|
|I second Stevens opinion, bad coil, unless you have a pertronix ignition, If you do, replace it with points 1st.|
|How does a bad coil, or bad points relate to the very high temp at the rear cylindar exhaust???|
|I think the high temperature at the rear exhaust may be a separate issue from the engine stalling -|
would an intake leak closest to #4 be causing a lean mixture which in turn would cause that exhasut to show hotter? Mr Eastman - how long before you can restart?
|It takes about 20 minutes before it will restart.|
I just had the engine rebuilt and have installed a new distributor (not Petronix). The plug wires are not mixed up. I think its the heat from the rear cylinder that is so much hotter than the rest that is making the fuel vaporize. I am going to wrap the exhaust manifold (even though already applied jet hot) and see if that works.
If I apply jet hot to intake manifold will it help if fuel prematurely vaporizes there?
Conversations I had with JetHot years ago the rep told me no advantage to doing the intake...so I did not treat. If your getting same spark to all 4 I would still suspect there is some kind of leak or restriction in #4. Somebody on this BBS years ago recommended stainless fuel lines over the rubber that was on my car when purchased. My bout with VAPOR LOCK was discussed at length on this post (should be in archives). Problem with mine was I followed ALL the advise and did it all at the same time so I don't really know what curred the problem. (I just knew I really did not want to come home on the rollback again!) Just about everything the experts here told me to look at ...looked wrong! (Carbs, water pump, fuel pump & lines, dizzy & wires, ect...I am still amazed the car ran at all)
What you describe here does not sound like the same "vapor lock" problem I was having though. Mine would run for hours just fine...it would vapor lock when I shut down for a short period and would not restart until it cooled. (as in by the time the rollback got me home). I still have a couple of "instant chill packs" in the car I was using to cool the rear carb down to restart. Never measured the temp like you did ...but I don't think I had that severe a difference front to rear.
Wrapping the exhaust, IMHO, could help but it's kind of a "band-aid" fix for a more serious problem and could cause more harm than good. I would be concerned about burning a piston with this kind of heat problem in #4. Not a fun thing ...melted #2 & #3 down in my car right after engine was rebuilt. (less than 300 miles). What process did your engine re builder use to clean the block? Maybe worth giving it a good pressure back flush in hopes to clear a blocked water passage? Most oil change places can do this fairly cheap and worth a shot before you have to "take her down". At least you'll get to "see" what was in there as most of those machines have clear reservoir to look at what came out.
Hang in there!
David 55 TF1500 #7427
|My TF still has the old school float depressors on the fuel bowl tops. These are often critisized for being dangerous etc (which they probably are)but they provide an absolutely instant cure for vapour lock on the very rare (heat wave) conditions where this occurs. I just turn the key on and depress the two plungers, the pump pressure blows the vapour straight out and I am on my way. They are also excellent for priming a cold engine the same way. Very little cranking required on a cold morning this way.|
But this may be getting a bit off topic now, I dont think it is the cause of the hot cylinder.
I still have the old SU "eyewashers" also,but learned all too late I could have purged, and been on my way!
Ed, Word of caution here...make sure you are very aware of how the drain tubes are routed before you "tickle the pins"! Don't think fuel vapors draining onto something over 500 degrees is a good idea.
You do have a fire extinguisher in the car ....right? You should.
|I will add again one more "vapor lock" experience. Like some of the above problems, the car would run about 15 minutes then stall and not run. 30 minutes later, it would start and run 15 minutes, etc.|
After doing everything from clothes pins on the fuel line to changing all the ignition components, I performed a fuel flow test on the fuel pump. Was looking for a certain amount of fuel in a specific time period. Set up the test with some clear tubing from the pump into a container. Pump flow tested good, but had a continuous stream of very tiny bubbles, which I was able to see by chance due to the clear tubing. Problem was a pinhole in the pump diaphragm. This pumped air was the source of the "vapor lock".
|D C Congleton|
|The hot cylinder may be caused by a sticking exhaust valve - quite likely on a rebuild, especially if bronze guides are fitted (incorrectly!). Also if the rocker bushes were not reamed correctly. And, if the valves - all or several - are doing it after it gets hot, it would account for the no run condition.|
So, tell us what exactly was done to the head.
Or, if the distributor is as many are, knackered, that cylinder may be getting a very late spark. Since you have a new one, I'd still check spark timing on each cylinder individually - manufacturing errors can really get you.
Wrapping the exhaust is not a good idea to cover up this sort of trouble, it will only get worse to terminal.
One thing you have not told us:
How many miles on the re-build?
IMHO : Go for the cheap/easy fix first!
Purge the fluids (oil-filter & cooling flush)
Add a pint of Marvell Mystery Oil to fresh oil fill. I have had really good luck with MMO for unsticking a valve in old engines that have not been run in years.
If you are near a small airport using airport transfers tonbridge seek out a mechanic and have him run an Annalise on a sample of you oil. Finding out what is "in" the oil after a rebuild if nothing else give you a sense of security.
These are fairly inexpensive actions when compared to "taking her down".
I would love to hear something this simple cures the problem.
Although you have a refurbished distributor, I thought I did too, and found that the refurbisher left me with an advance plate that would lock up when the center screw was tight, and a set of bushings that would not hold my shaft steady. A complete rebuild by Jeff Schlemmer of Advanced Distributors cured those problems.
I completely concur with Joe that you will find that the hot exhaust gas temperature in 4 is unrelated to the stalling problem, unless as Fletcher suggests every valve is sticking in progressive degrees. I also concur with Joe when he suggests an immediate spark check when the car shuts itself down, to eliminate an electrical problem.
JetHot should be more than sufficient to prevent heat soaking and adding a wrap to the manifold makes no sense to me. In fact, I run completely stock pump, fan, and radiator and on a 90 degree day I might get 90 degree C on my temp gauge. The only changes to my car is JetHot, the timing at 11 BTDC, an 8.5:1 compression ratio, bronze guides reamed a couple thousand oversized, and I drilled an additional hole for cooling in the forward water jacket as suggested by the TC site. The extordinary means you have undertaken to solve a hot running problem may not be necessary if the head is checked and you confirm that you have a properly operating ignition system. It almost has to be one or the other.
"not hold my shaft steady"....hmmm I'm not even going there! LOL
|I doubt that a sticking valve is causing the problem you're having with the #4 cylinder. Both times I had that happen I snapped the rocker arm for the valve that was sticking.|
I agree with Joe Buckmiller above...look for a leak around the intake of #4 which could be leaning out your mixture.
But...that still won't explain why you stop running after 15 minutes.
Have you checked your fuel pump when this happens to make sure it's still pumping? If not, next time it happens immediately remove the fuel line to the carbs and turn on the ignition to make sure you've got fuel coming out. Uhhh, carry a can/bottle with you to catch fuel. If the pump works then I doubt that it is a fuel/carb problem. An overheated coil could well be the problem.
Valve(s) doesn't need to be "stuck", just lazy in closing.
Correct me if I'm wrong, but if the intake valve is slow closing you'll be backfiring out the carburetor...if the exhaust is slow closing the cylinder won't be firing effectively, if at all. I don't see how either will cause the cylinder to run hot.
If the exhausts are not quite getting closed, say by less than the amount of the valve clearance, it will leak a lot of heat, but still run up to a point. That point would be when the balance between enough and not enough compression is passed. If #4 is worse than the rest, it will get hotter (and tighter) faster, while it still runs on the other 3 cyl. Remember that his initial complaint is an apparent "vapor lock" condition, which is not likely, it's really engine shutdown after it gets hot. Entirely in keeping with exhaust valves hanging in too tight guides, and going away after they cool. It does not sound like a general heat problem, but does make sense as a local valve related one.
|I had vapor lock a few years ago on my TF -had to stop every 15 minutes or so back to NJ from INDY. Turns out my mechanic had put a new fuel filter on under the engine. He still swore that could not cause it, but I took it off and it ran fine.|
|K E MURPHY|
I'm sorry, I just can't agree. A sticking exhaust valve will make an engine miss but won't cause it to shut down completely. I've had a valve seize completely, snapping off the rocker arm, and the engine, although noisy, kept right on trucking down the road.
|Yep, you're right Gene, and so is Fletcher. A sticking exhaust valve will hamper heat transfer, cause high EGT values and can result in the damage you speak of. But if one valve (#8) is too tight at a reasonable head temperature, others (#1, 4 and 5) could be too tight at more elevated temperatures, until they all stick as the temperature increases due to poor heat transfer. We don't know at what point of engine warm up Ed made his manifold measurements.|
However, a bad coil (heat sensitive) will shut off the engine as a separate problem. I believe Ed has two problems, with only a mild relational aspect to them. At the point of shut down he needs to verify spark or the lack there of.
You know, my tools only extend to the end of my arms, and Ed is many miles away. Until someone with some diagnostic ability looks at the engine, we are all guessing. Our hope here is to lead Ed in a successful path of analysis of determining the problem by offering likely causes. Throwing heat related solutions at a simple engine such as an XPAG/XPEG will mask, but not solve the problems.
This thread was discussed between 02/08/2009 and 06/08/2009
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