detinating inside the head,

[pauli]

hi,

just had the engine out of my 88 turbo to find that it had melted two pistons and damaged the head. I have just had a brand new engine put into the car, but the garage that have fitted it tell me that there is something wrong with the vaccum, as when it is on boost it is supposed to retard? or something like that. Anyway i don`t want to get a melt down on the new engine, has anyone experienced this before and what the possible causes may be? is it to do with the timming? or the hoses not being in the right place.

thanks all

[jk1]

Sounds fishy to me. If you paid them to install it and they were supposed to know what they were doing then I would think they should have made sure it was running to spec before they gave it back to you.

How long between fitting the new engine and the failure? Was the engine brand new, rebuilt, used?

No expert here but I'm pretty sure if the wastegate which is vacuum operated wasn't working properly it could allow it to overboost, although the ECU is supposed to prevent that from happening.

FWIW, I had an overboost condition one time when the vacuum line was inadvertently left unplugged.....It was pushing 21-22 psi according to my digital boost gauge.

Sorry for your trouble,

Jim
95 S4s

[pauli]

hi Jim,

i have not yet run in the new engine (which is a brand new one) they have just fitted it in the garage, but they are not sure what the cause may have been on the old engine which is the one that melted. I don`t want to run this car with the new engine as i don`t want the same to happen as i have just shelled out a bucket of cash for it. Do you know what the boost is supposed to run at? is it around 7

>> Edited by pauli on Friday 20th August 19:03

[lotusguy]

Hi,

High combustion temps are often caused by a lean condition. This can be improper fuel metering, or any number of possible leaks.

Since they indicate the vac pressure was low, this too can be the result of leaks. If the fuel metering was to blame, vacuum pressure would be less effected I should think.

Look for vac leaks, head gasket, intake manifold and so on. Hope this helps.
Happy Motoring! ...Jim'85TE

[Esprit2]

pauli said:

(Snip)... the garage that have fitted it tell me that there is something wrong with the vaccum, as when it is on boost it is supposed to retard? or something like that. (Snip)...

Just addressing that statement alone, the distributor is equipped with a vacuum diaphragm that has two hose spigots, one on each side (push-pull). If the hose connections are reversed, either at the diaphragm or anywhere else in the system, the advance function can be reversed.

That could result in an inappropriately advanced ignition timing under boost conditions. That could lead to detonation and excessively high MEP (mean effective pressure). That's more likely to cause physical damage (bend/break something rather than burn it), but it could also result in higher combustion temperatures and melt down.

Page 43.03C in the 1988 Service Parts Manual is a detailed, pictorial diagram of the vacuum system components and hose routing. Refer to that as a road map and trace every inch of hose. Inspect it for cracks and loose connections, and replace any suspect bits. Make sure all the hoses go to the right places and that connections have not been reversed.

Of the two connections on the distributor diaphragm, the outer one (away from the distributor body) is all black and goes to the center port on the Thermal Ignition Valve (TIV). The inner one is black with a blue stripe and goes to a "T" fitting in an intersecting hose… also black with a blue stripe. The short end of the intersecting hose goes up to a vacuum spigot on the bottom of the #3 throttle port. The longer end of the hose goes to the right side of the car, through the right engine bay wall, and up to the top port of the Idle Up Vacuum Solenoid Valve.

There are more connections, solenoid valves and thermal switches. You really need to have the manuals to check it all out.


On the 1988 models, the vacuum system has nothing to do with boost control. A boost pressure hose runs directly from a fitting on the turbo outlet neck to the wastegate diaphragm. No electronic or vacuum diaphragm intervention. If the wastegate sticks or if the boost sensing hose is split or disconnected, then the boost can go sky high.


If the pistons were truly burned rather than physically damaged, then you should also verify the fuel injection system. A lean mixture due to incorrect metering, dirty fuel injector, a stuck bypass valve or a large air leak could all contribute to a lean mixture that would burn very hot… potentially burning a piston.

How much was replaced with the "new engine"? Was it an all-new crate engine, a used engine, or a rebuild? If it was a complete plug-n-play replacement, all the bits directly associated with the engine should be new. You should be able to scratch them off your list of culprits.

Many of the vacuum hoses would have to be disconnected during the engine transplant, and there's always a opportunity for a mistake in re-connection everything. There's also a chance of cracking or tearing a hose in the process. The Bosch metering unit would probably carry over undisturbed and that could still be a problem.

The Bypass Valve is controlled by the Throttle Position Switch (TPS) and an RPM Relay. If the TPS is improperly adjusted, if the micro-switch inside the TPS is sticking, if the relay contacts are sticking, or if the Bypass Valve itself is faulty, an un-metered flow of air could be flowing into the engine at the wrong time… causing a lean mixture.

There are a lot of "IF's". What you need is a set of manuals, and a mechanic who knows the car. At the very least, a mechanic familiar with the Bosch K-Jetronic fuel injection system (Mercedes, Porsche, Audi, VW, others… very common) and a willingness to spend some time with your manuals without billing you an hourly rate for his education.

Best of luck,
Tim Engel
Lotus Owners Oftha North
Minnesota, USA

[pauli]

hi,

thanks for the reply, the engine is a brand new one out of the crate the only parts that came off my engine were the carbs (rebuilt) and the distibutor. As the engine is a s4 with fuel injection. So i didn`t want to go into the process of putting all the extra parts in for a staight fit with the injectors. The garage that have fitted the engine are supposed to know what they are doing, but it seems as though they are a bunch of monkeys as they can`t figure it out. I will get the car back from them on monday, i have a set of manuals so i will have a look through and see if all the pipes are in the correct place, (i have also replaced them all for new samco hoses), then like you say i think it will be off to someone who knows what they are doing with it to set it all up ok, or i`ll be waving goodbye to another £6k very quickly. I live in stoke-on-trent, if anyonen knows someone in this area who has the knowlage to check it for me i would be greatful for the contact. Well in any case i shall make the parts fair at stonleigh this year so hope to see you all there.

thanks

pauli

[grk]

Hi Paul,

Garry here, I pulled apart your old engine yesterday, there were certainly some issues with it:

two of the pistons are heavily detonated in the top ring groove, the head is also heavily pitted on the exhaust side of two chambers.

There also seems to have been oil and water mixing issues as the oil in the sump was a milky grey mess. I know you mentioned the engine had been rebuilt in the past before you owned the car and I suspect some of the blame may lay with who ever did the work: as far as I can tell there was no sealant on the bottom of the cylinder liners, which would explain the oil / water mixing.

The cylinderhead studs were not changed to the newer "dimpled" type that need to be fitted with the new gasket and I suspect may have lost tension under high boost conditions allowing the head gasket to blow.

The final thing we discovered if you recall was an absence of gaskets between the carbs and the plenum, this would create an air leak and a lean condition which can cause detonation.

It's important that there are no air leaks on the induction system and vacuum lines; they need to be carefully checked for leaks as detonation will kill an engine pretty quickly.

Tim Engel has written a very helpful reply above, but you need to be aware that Tim's comments refer to the US fedral equipped cars that were fitted with Bosch K jetronic fuel injection, us guys in the UK had to soldier on with dellorto carburettors for a couple of years longer than the federal cars, and your car is one of the late dellorto equipped HC cars.

Your block is now stripped down and cleaned so next time you are passing this way feel free to call by and collect.

[pauli]

hi gary,

well i shall get this all checked out once it returns to me, but i think the main causes will have been the vacum hoses, i shall also throw on a new dizzy for good measure. Will you be going to stonleigh this year? as i can collect the head from you there if that is o.k.

thanks all for your help

pauli