Intake System Pressure Testing

The 951's great acceleration comes from the boost system. Turbo charging is a fuel management challenge because we are expanding the range under which the fuel to air ratio must be managed.  Generally, the amount of air being introduced into the system is measured by the air flow meter (AFM). The stock AFM is essentially a barn door device.  As air is drawn in, the door swings open. The angle of the door is electrically measured and represents the amount of air coming in. The air is then compressed slightly (to a max of about 20 pounds) and is pushed through the inter-cooler and into the intake manifold. From there it is routed to the cylinders. 

Fuel is regulated by the engine control computer (DME) and a pulse stream is sent to each of the 4 fuel injectors. A longer pulse, holds the injector open longer, resulting in more fuel. The pulse length is adjusted to maintain 1 part of fuel for every 14 parts of air. I've made this sound pretty simple; actually, engine RPM, throttle angle, oxygen in the exhaust stream and engine temperature are all variables used to regulate the fuel system.  To assess all of the variables, the computer uses a series of digital maps.

If the  measurement of any of these variables is not valid, then  the process of managing the air to fuel ratio is corrupted. Usually the result is an overly rich mixture which is bad for the efficient fuel burning, and peak power. It can also damage your catalytic converter.

Everyone who owns a turbo learns one lesson pretty quickly. If a leak develops along the intake system, a pronounced stumbling occurs when under boost.  This can easily happen if you forget to get those intercooler pipe clamps tight. This stumbling is happens because the fuel management system is expecting more air to be delivered to the cylinders than is actually being provided.  A reduced amount of air means an overly rich system, which means poor peak power.

There is generally two ways to find leaks in the intake system. You can carefully spray carb cleaner or LP gas on the hose connections, if there is a vacuum leak (not necessarily the same as a boost leak), the cleaner will be drawn into the idling engine and an increase in RPM can be easily detected.   While this method will work, it is mildly dangerous - many engine fires have been started with this method.

A better way is to have the engine off and  pressurize the system with compressed air and simply listen for the hissing sound of air leaks.  This approach is not only safer (the engine is not running) but a bit more reliable too. So, how do we introduce compressed air into the intake system? With an adapter, of course. Does such an adapter exist? Well, yes and no. To my knowledge, there is not a manufactured tool for this job.   

Above you can see one method for making a pressure adapter. Yes, that is a 944 fuel filter. It is fitted with a simple air hose quick disconnect fitting. With this fitting, I can apply regulated air pressure and then listen for leaks.  

This method will work but.... a couple of problems. First the fuel filter really doesn't fit well and no matter how I tightened the clamp, I still had leaks there. That big leak means my air compressor is cycling on every few minutes and just when I think I might have a bead on the air leak the noise form the compressor wreaks my focus and I stand around and wait while it charges. Second, this method pressurizes the turbo and the j-pipe. The turbo may leak and yet be difficult to isolate and the J-pipe is not designed for pressure and may leak or even burst.  

A better method is to pressurize on the downstream side of the turbo. To do that I came up with this boost pressure tester. More info on it here.  This tester works great. It hooks up easy - in about 2 minutes. The gauge on the my airgun tells me how much pressure I'm about to put into the system.  The trigger gives excellent control over the air flow and I can isolate those leaks much easier.  I also connected an old fuel pressure gauge to the FPR's hose so that I could monitor the pressure and get an idea of the size of my pressure leaks. One small note: using the boost tester means there is one hose that is not pressure tested. That hose, turbo-to-intercooler-pipe must be inspected visually.

 It turns out that I had several leaks and to help know exactly where the problem was I used a spray bottle of soapy water to help identify exactly where the leaks are. The leaking air, makes for bubbles and leaks are easily isolated. Above are some of the surprise places that I found to be leaking.

Vacuum Leaks

The problem with this testing approach is that many of the leaks are under the intake manifold. When you remove the manifold, you will find a "rats nest" of hoses. Their proximity to heat and age make them candidates for replacement. Before you starting ripping those old hoses out though, you might consider what you're getting into. 

At the center of the mess is a "console" - 3 steel tubes welded together. Steel was probably a good choice here because of the proximity to the hot  turbo. The steel doesn't typically fail, the rubber that is attached to it does.  In classic Porsche style the P-boys made this little assembly as a single item, meaning that it is not meant to be repaired. No problem, a quick $450 will get that item replaced. The other rubber pieces are also a bit pricey and come in right at $200. 

Now I love my 951 but $650 has me considering my options. Maybe there is a worthy alternative.  This subject has come up several times in the Porsche forums and it seems there are a couple of different ways to go at this problem.  I'm not going to try to write the end-all analysis of this topic but I will give you the little bit that I've learned.

Like everything I like to start with a general assessment and move on from there to the specifics. In the general category I found that I had several small pieces of silicone tubing being used for vacuum lines. This colored tubing is very popular these days with anyone considering an engine dress-up. That's fine by me but sometimes the favorite tubing isn't really well suited to the 951. I found that mine would actually collapse when vacuum was applied. I observed this with the engine idling, see above.  Replacing it didn't help my overall problem though.

The first thing I needed to think about is the venturi. This little device is meant to enhance vacuum boost pressure (for the brakes) under part throttle situations. Some Porsche owners delete this item. I am reluctant to do that, but if you do decide to go with the venturi-delete idea, there are some special formed hoses that you can get from your dealer and I'm told they are reasonably priced ($75 for the pair). 

So, with the intake removed, I set about inspecting each of the hoses in hopes of 1) finding them to be ok (I wish); 2) finding an appropriate replacement; and 3) restoring peak power. As you inspect your hoses, remember that these little guys are not only old but have been subjected to pressure changes ranging 3 times that of hoses for normally aspirated engines. Also, the connections may hold a vacuum but not hold under boost pressure. Keep that in mind if you're testing with a vacuum gun. Loss of boost means loss of power (duh). 

In the process of looking for leaks, I found this wiring problem, I was all excited that maybe this had contributed to my rich running, stumbling under boost problem. This connector is for one of 2 engine temperature sensors. This particular one tells the DME when the engine is warmed up. I corrected the wiring problem but I'm not sure that it really was a problem. Even so this is an easy item to check while the intake is off, so look for it.

In the above picture, you can see what some have referred to as the "rat's nest."  At the center there is the console assembly (upper right) that I mentioned earlier. Replacing the rubber lines (for the console) is not really a problem as all but one of them is simple 5/16 fuel line. To replace this rubber, I removed the assembly and cut the connector collars with a thin 4" abrasive disk. 

The one formed hose  is a bit of a challenge as it fits into a crowded area near the TPS. I went in search of a hose for that one and after searching through the back room stocks of several auto parts stores found one that fits. It is a block heater hose for a 79 Honda Accord. If you go this route, the Honda piece has a simple 90 degree bend and will have to be trimmed in length by about 2 inches. It makes for a perfect fit though and looks very near stock.

One small cautionary note, do be careful in handling the cycling valve. It's plastic construction becomes quite brittle with age and the fittings are easily broken. Also, removing the intake pretty much means replacing the the 4 intake-to-head gaskets (they cost a $1 ea). The forward one must be installed up-side-down, else the injector is blocked. I don't believe Porsche uses gasket sealer but I want a better seal without torquing the intake really high so I use one of the new sensor safe silicone sealers. It works beautifully and comes off easily at replacement time. 

Next up, are all the other hoses, I believe there are 6 of them and cost about $150 all together. One of them contains a restrictor. I needed to replace a lot of clamps. Many of them are the one-time-use clamps, put on by the factory. I like these except they they, over time, loosen up and without the crimping tool are hard to tighten up. Some people have a strong preference for the various OEM screw clamps and I do like the original look. Beyond that though I prefer stainless material over some sort of the coated OEM clamps. I like the rolled edge used on some of the Norma clamps. 

For some contrast, a pic of the NA's hoses. I didn't mention the brake vacuum line earlier and this NA photo shows it well (the same line exists on both cars). It is 15/32's - seems like a strange size for the P-boys to have used but that is OEM and it is marked as 15/32s. Weird.

The one hose I haven't mentioned or shown is the one going from the cycling valve to the wastegate. It is also 5/16ths ID and about 3 feet long. It is often dried out and stiff as is it is subject to quite a lot of heat. It is difficult to change, so plan accordingly.