w108 4.5 cooling issue/introduction - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 11-13-2015, 08:31 AM Thread Starter
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Date registered: Nov 2015
Vehicle: 1972 280sel 4.5
Location: Atlanta, GA
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w108 4.5 cooling issue/introduction

So I am new to this forum, but I figured I would join seeing as there are people here who know more on my car than I do. My great grandfather bought a pale yellow 1972 280sel 4.5 brand new. Unfortunately shortly thereafter he suffered from ALS and was no longer able to drive. He cared for the car meticulously as did his family after his passing. The car received little use (fewer than 50k on the car) and the family members who had the car did not want it to leave the family. They called us (the other family members) and asked if we were interested. My father and I spend our time restoring and enjoying motorcycles and other automobiles including my 75 cb750 cafe his 82 xv920 street fighter, a 48 willys cj2a, a 52 dodge m37, a 49 flexible clipper (all of which we restored ourselves so we are not new to cars or bikes), and a 96 911 993. Its no wonder we wanted the car and we are now in the process of restoring it including paint, suspension, interior and mechanical work. After I graduate college the car will become mine and needless to say I am enthusiastic.
On any reasonably hot day the car tends to run very hot. If I turn the car off and attempt to restart it shortly after it turns over but refuses to fire. eventually it will run on a few cylinders, and once it gets moving it works fine. I am assuming this is a cooling issue so what is my best course of action, new radiator, new fan, both? Also any other advice anyone has for a new w108 owner.
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post #2 of 7 (permalink) Old 11-13-2015, 05:23 PM
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Hi Kase, Welcome and best of luck with the car. The 4.5's are known to run hot and have vapor lock issues with fuel supply during hot starts. My 4.5 has had both issues. I went through my entire cooling system, replacing just about every part/sensor/switch and getting the radiator re-cored. My mechanic also installed a switch that allows me to turn on the auxiliary cooling fan independent of the thermo-switch. We did a citric acid flush of the system and installed a larger auxiliary fan from a later model car (I think an R107 560SL but it could have come from a W126). That fan makes a big difference on hot days in stop and go traffic. My other discovery was that the car doesn't like any additional timing advance. My mechanic prefers a little extra advance for better performance but in my experience the extra advance causes the car to run hot - particularly in stop and go traffic. The vapor lock issue is one that I still experience occasionally but it hasn't been so bad that I've tried to to remedy it. I've read other threads on the MBCA site that indicate Mercedes issued a replacement head with special heat shields designed to help overcome the vapor lock. I believe these are no longer available. I'm in the process of having my 300SEL redone (has the 3.5 engine) and we rebuilt the engine completely. Although the engine was in great shape overall, a few coolant passages in the heads were clogged with a build up that was restricting flow. I'm eager to get that car back on the road to compare the newly rebuilt 3.5 to my all-original 4.5. Again, good luck and keep posting. Dave
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post #3 of 7 (permalink) Old 11-14-2015, 06:46 AM
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Welcome, sound like a nice car. Please post some pictures of it. I dont know that engine specifically, but does it have a fuel accumulator? That is designed to help with hot starting by keeping fuel preasure up. The gas today doenst help if it has alcohol in it. try insulating all fuel lines and heat coming from the exhaust manifold up.
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post #4 of 7 (permalink) Old 11-14-2015, 04:20 PM Thread Starter
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Im not sure if it does or not but thank you for the advice, I'm hoping a nice big electric fan and a re-core will solve the problem or at least make it less severe because at the moment it is pretty bad, also the car is in the process of being restored but i will post pictures as it makes progress
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post #5 of 7 (permalink) Old 11-14-2015, 08:55 PM
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The hot start issue is a known pain in the ass. You'll better spend your money by installing a bypass to run the fuel pump continuously without starting the engine. It sends cool gas to the engine and defeats the vapor lock.
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post #6 of 7 (permalink) Old 11-16-2015, 07:58 AM
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Manifold Heat wraps should help in keeping underhood temperatures low and preventing vapour lock .
My 126/300se has fuel passing through a freon return pipe exchanger to cool fuel,I intend to incooperate it in the 3.5 as well.
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post #7 of 7 (permalink) Old 11-17-2015, 07:36 AM
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The hot start problem is a well known issue and mainly is a result of the low fuel pressure the D-Jet injection system uses (29psig). You can increase this pressure but then the fuel mixture controls need adjustment. For idle mixture, the adjustment is on the ECU and for load mixture, on the Manifold Pressure Sensor. But to adjust these, you need a means of measuring %CO in exhaust (or AFR). You can't do it blind or by feel. I installed a permanent wideband O2 sensor with AFR gauge on my own car. Portable units are available.

To overcome overheating, I would first look at timing. The NA specs call for less advance than the Euro cars. This for emission reasons. Try setting the idle timing at about 5deg BTDC. Then rev engine to 3000rpm and check how far it advances. It should get to about 27-30BTDC. If not, turn distributor more counterclockwise. If you don't see that sort of advance, the internal centrifugal weights in the dizzy could be sticking. Running retarded causes overheating.

The hot start problem is not likely due to vapor locking in fuel rails. It is most likely caused by fuel flashing to a mix of vapor and liquid as it passes through injectors. This reduces the injector capacity so cylinder only gets a portion of what ECU is trying to provide. As a result mixture is way too lean, rpms drop and engine cuts out. Keeping tank full and running fuel pump several times by switching key on and off can help get some cooler fuel into rails. A switch to bypass the fuel pump relay would do the same thing. But this relies on fuel in tank and supply line being cooler. Not always the case when parked on hot tarmac!

I tried all sort of "fixes" including some of those mentioned for the hot start problem. But in the end, more advanced timing, 32-34psig fuel pressure and mixture adjustment resulted in no further problems.

There is a section here on the 107 forum that has a lot of info on the Djet system. It is in Encyclopedia Germanica in 107 forum stickies under D-Jetronic. The D-Jet is a unique system. It can work well, but often not understood.

Have fun with your new project!

1972 350SL 4.5L - MBGraham, near Kingston, Ontario

Last edited by MBGraham; 11-17-2015 at 09:25 AM.
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