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Bosch CIS Hot Start Problem


One of the most common questions on with the Bosch CIS fuel system concerns the so-called “hot start” problem and how to fix it. Unfortunately, there is not a single cause but rather as many as 10 possible causes and it can be easy to spend a lot of time replacing different parts and getting nowhere. For this reason, a common modification, known as the “hot start fix” is often applied to work around the problem without actually solving it; however, this isn’t always the best idea. I’d recommend reading the Bosch K Jet-tronic “Workshop Manual” for Porsche cars first if you are unfamiliar with the operation & components that make up the CIS system.


In a nutshell, the car won’t start when it’s hot. It starts fine when it’s cold and runs perfectly until the engine is turned off and but when you try to restart the car after a period of time, it turns over on the starter but refuses to fire. It may start after turning over for some time, or it may have to be left to cool down altogether before it’ll start again. It can be an extremely frustrating problem.


The basic cause is very simple: vapor in the fuel supply to the engine. The fuel system is designed to keep gasoline in liquid form until it reaches the injectors and enters the inlet manifold, and the various components of the system regulate the air/fuel mixture to keep it at the correct ratio to run the engine. If the fuel is allowed to vaporize too early, the regulation cannot be maintained and the engine doesn’t run. Until this vapor is replaced by liquid fuel, the engine won’t start.

Under normal circumstances, the fuel system uses a simple technique to stop the fuel turning into vapor: pressure. When running, the pressures in various parts of the system range from about 30 psi to 75 psi, more than enough to keep the fuel in liquid form.

However, if the engine is switched off and the residual pressure is not maintained, the heat inside the engine bay warms up the fuel and allows it to expand into pockets of vapor.

Confirmation that this is indeed the cause of the problem can be determined by measuring the Residual Pressure (the fuel pressure in the system after the fuel pump is turned off) in the fuel system. The Porsche specifications are after turning off the car for 10 minutes the residual pressure should be above 25psig, after 20 minutes the pressure should be above 22 psig and after 60 minutes the pressure should be above 14.5 psig.


The intent of this document is to provide a systematic approach to identify which component(s) is the root cause via test methods.    

There are ten potential causes:

Warm-up Regulator (WUR)

Close the valve that is part of the fuel pressure gauge between the WUR and the fuel distributor.  After re-pressurizing the system, if the Residual pressure is now within spec, then the either the WUR or the gas cap is at fault. If after checking the “Leaky gas cap” step and the cap is determined to be OK, then the WUR is at issue.   The WUR will need to be refurbished

Leaky gas cap

Visually inspect the gasket on the gas cap for any cracks or tears in the gasket.  If gasket is compromised either replace the gasket or the gas cap and then re-run test #1 to confirm the residual pressure is within spec.   

Bad or leaking fuel accumulator(s)

If the diaphragms in the accumulator(s) fail, they are designed to create an external gas leak.  Remove the small screw(s) at the bottom of the accumulator(s).  Visually inspect for any leak immediately after turning off the fuel pump.  Replace is there are any signs of leaking.

Fuel pump check valve leaking

Depending on year & model the check valve is either internal to the fuel pump or external to the pump.  The check valve is designed to prevent flow returning the to the gas tank when the pump is turned off.   After turning off the fuel pump, immediately clamp the hose on the outlet of the fuel pump using something like a long nose Vise Grip.  WARNING: DO NOT CLAMP THE HOSE WHILE THE FUEL PUMP IS RUNNING. A HIGH FUEL PRESSURE SAFETY HAZARD WOULD EXIST.  If the Residual pressures improve, then the check valve is bad.  If the car has an external check valve then it needs to be replaced.  If check valve is internal to the pump, then replaced the fuel pump or add an external check valve (Bosch 1-587-010-536)

Air/fuel adjustment set to be too rich

If the air/fuel adjustment is set too rich, then the slits in the distributor plunger will be opened when the engine is turned off which will reduce the Residual pressure.   Remove the banjo bolt screw for anyone of the fuel injector line from the fuel distributor.  With the fuel pump manually bypassed to run, if there is fuel coming out of the distributor outlet then the air/Fuel adjustment was set too rich.  Turn the air/fuel adjustment screw CCW until the flow stops.  Then turn the air/fuel adjustment screw an additional 180 deg CCW.

Fuel distributor plunger O-rings leaking

The plunger has O-rings around the slits openings.  If the O-rings fail to seal, then fuel pressure can escape, causing low Residual pressure.  Remove all of the injectors, leaving the fuel lines attached.  Place the injectors into individual clear plastic containers.  Manually bypassed the fuel pump to run and after 30 seconds, then turn the pump off.  Fuel should stop flowing from the injectors very quickly after the pump is turned off.  If none of the injectors are leaking then both the distributor and/or the injectors are not causing the issue.

If they all are leaking then make sure the air/fuel adjustment is not set too rich before proceeding.

If some of the injectors are leaking and some are not, then jot down which are.  Say for example injector for #1-cylinder was leaking and for #2-cylinder was not. Swap fuel lines at the fuel distributor.  Connect the #1-cylinder line to the distributor port that originally feed #2-cylinder.   Connect the #2-cylinder line to the distributor port that originally feed cylinder #1.  Watch again what happens again after turning the fuel pump on & off again.  If the leaking injector is now that one connected to the distributor port that originally feed #1-cylinder, then the O-ring inside the fuel distributor needs to be replaced. The fuel distributor should be refurbished.  

Leaking injectors

If the leak moved to the line that is now connected to the distributor port that originally fed the #2-cylinder, then the injector is stuck open and will need to be replaced.  

Cold start injector leaking

Remove the cold start injector from the intake manifold with the fuel line attached but electrical plug disconnected.  Place the outlet of the valve in a clear plastic container.   Connect 12v directly from the battery to the valve.  The valve is polarity insensitive so it doesn’t matter where the + and – is connected. With the fuel pump manually bypassed to run. There will be fuel coming out of the valve. Remove the 12v power to the valve, and fuel flow from the valve should immediately stop.  If the fuel continues to leak out the valve then it will need to be replaced.

System pressure regulator O-ring leak

Internal to the fuel distributor is the system pressure regulator. It has an O-ring that seals to prevent flow back to the fuel tank.  If the O-ring leaks then Residual pressure will be low.  Turn the fuel pump off and immediately clamp the hose that feds back to the gas tank using something like a long nose Vise Grip.  WARNING: DO NOT CLAMP THE HOSE WHILE THE FUEL PUMP IS RUNNING. A HIGH FUEL PRESSURE SAFETY HAZARD WOULD EXIST.  If the Residual pressures improve then replace the O-ring

Supplementary start valve

If this valve is leaking it will bypass flow around the WUR to the return line going to the gas tank.  This will reduce the residual pressure.  It will also reduce the cold & hot control pressures making the car run rich.  Unfortunately, the lines connected to the SSV are rigid lines so there is no easy method to verify that when closed its not leaking.  If both cold & hot control pressures are within spec, then the SSV most likely would not be an issue.

Tech Articles – Working on Your Porsche

Ever wonder if it is possible to work on your own Porsche and save 100’s if not 1000’s of dollars with the gratification that you did it yourself? We’ll try to build up this section with “how to” articles from simple oil changes to upgrades and modifications. Got an article of your own? Send it to webmaster@cnypca.org and we’ll publish it!

IPD Plenum Installation on a Cayman 987.1

So you want a little more performance out of your 2006-2008 Cayman? (or Boxster though the procedure is slightly different). The updated intake plenum from Innovative Pro Design might be for you!

The plenum shown in this installation procedure should provide about 15 extra horsepower and 20 lb-ft of torque. This is done by eliminating the restrictions of the stock, divided intake plenum, as well as relocating the throttle body right next to the plenum.

In addition, this plenum helps to straighten the path of the driver’s side shifter cable and simplify the AOS (air oil separator) inlet for those who may want to install a catch can to prevent oil from being introduced into the intact tract. The stock shifter cables are known for failing around the 80k mile mark, but more of that in another post. The air oil separator will also be a separate post.

To acquire an IPD plenum, you can either contact me (webmaster@cnypca.org) or direct to https://ipdplenums.com/products/plenums/caymans/9871-caymans. Note that they also make performance improving intake plenums for other Porsches as well!

Recommended tools required for the installation include:
* 7mm socket and nutdriver handle to remove and replace hose clamps.
* 10mm socket with a universal joint adapter and socket to loosen the nut holding the stock manifold in place
* An E10 socket or nut driver for Throttle Body, oil filler tube bracket and factory plenum hold-down nut.
* Torx 30 screwdriver to remove the engine cover.
* Large Flat head Screwdriver to assist in gently removing the stock intake boots
* Pliers and/or Channel Lock pliers for vacuum lines
* Hose cutter or razor blade to trim the stock vent hose

1. Remove luggage net and carpeted engine cover. This requires the carpet and attached foam to be bent in order to clear the outside tabs. It is recommended to begin the removal process at the front of the cover where the tabs are smaller. Now remove 5 bolts that secure the engine cover with Torx 30 screwdriver.

Carpeted Engine Cover
Carpet removed exposing aluminum engine cover held in place with five Torx T30 bolts
Aluminum engine cover removed exposing engine

2. Disconnect crank case vent breather lines from both sides of the factory plenum and where they converge at the oil separator. Remove entire breather line assembly. Note these clamps are“Finger” squeeze type clamps that lock into place at the top and bottom of the clamp. You may need the gentle persuasion of some pliers to squeeze the clamp together. Note that the clamp is squeezed at the hash marks ONLY! They will slip right off when this is done right.

Removing squeeze clamp from the AOS (air oil separator)
Removing one of the two squeeze clamps from each side of the stock intake manifold

3. Remove 10mm nut that connects factory plenum to support bracket located next to the engine hoist support.

10mm nut holding intake to support bracket

4. Loosen all 4 hose clamps (red arrows) around the rubber boots that connect factory intake plenum to intake manifold utilizing the 7mm socket and nutdriver handle. Make sure to loosen clamps as much as possible without allowing the clamps to come apart. Leave clamps in place.

5. To remove factory plenum slide the rubber boots outward (away from the plenum) onto the intake manifold as far as possible (green arrows). Try to push the boots further onto the manifold towards the outside of the car to help ease factory plenum removal.

Loosen hose clamps (red arrows) and slide manifold boots towards the sides of the car (green arrows)

6. Loosen hose clamp on ribbed intake hose that attaches throttle body to MAF (Mass Airflow Sensor) with the 7mm socket and nutdriver handle. Utilize a flat bladed screwdriver to gently loosen the boot from the MAF. The MAF is located down and to the left when looking at the engine from the rear.

Loosening the clamp and boot from the MAF housing

7. Remove the factory plenum by lifting plenum upward. The plenum will still be attached by two vacuum lines on underside of plenum. First disconnect the vacuum line that attaches to the factory vacuum actuated valve. Disconnect this line by carefully pulling so as to avoid disconnecting vacuum line from opposite end deep inside the engine compartment.

Gently wiggle the stock intake manifold upward free of the manifold boots while reaching underneath to remove the actuator vacuum line
Second vent line still attached. We’ll take care of that in a moment
First vacuum actuator line. This gets plugged with the IPD supplied plug in the kit.

8. Disconnect throttle body from factory plenum by removing all four E10 bolts (red arrows). Then either lay the throttle body to the left side of the motor or disconnect the throttle body wiring harness by squeezing the sides of the connector (green arrows), gently wiggling the connector out and set it aside.

E10 bolt removal (red arrows) and throttle body harness plug squeeze points (green arrows)

9. Now disconnect the second vacuum line situated on the underside of the plenum. Slide squeeze clamp off nipple with a pair of pliers and slide hose off nipple. This may require the use of Vise-Grips to loosen up the hose before pulling it off.

Using a pair of Vise-Grips to remove the second vent line hose clamp

10. Remove the clamp that attaches the crank case breather corrugated plastic vent line to the engine block so as to provide additional room for the IPD silicone hose (red arrow to the left). NOTE: these vent lines can become very brittle and possibly crack/break when moved around due to years of extreme temps and repeated heat cycles. If the vent line cracks or break the replacement Porsche part number is 987.107.246.01

11. Remove the rubber mount that previously supported the factory plenum, located next to the engine hoist bracket and crank case breather vent line.

12. Remove the two E10 bolts that hold down factory engine hoist bracket (two red arrows to the right). This will provide additional room for routing of crank case vent line. Keep engine hoist bracket stored where you will not forget its location.

Vent hose clamp and engine hoist bracket removal. Replace the bolts after removing components.

13. Remove factory rubber ribbed hose that connected to MAF (Mass Airflow Sensor). All hose clamps will be re-utilized.

Remove MAF ribbed hose by loosening hose clamp. Retain all hose clamps.

14. You are now ready to begin the IPD plenum installation. First, plug vacuum line that previously connected to factory flapper valve if you haven’t already done so. Use small provided plastic line plug. This line is no longer necessary with the IPD plenum.

15. Reinstall the throttle body onto the IPD plenum. Be sure that throttle body is positioned so that the black plastic portion of the TB is situated to the left (driver’s side). Torque the four E10 bolts to 6 ft-lbs. Remember to plug the throttle body harness back in if you removed it!

Throttle body installed on new IPD plenum. Torque the four E10 bolts to 6 ft-lbs

16. Reconnect vent line with squeeze clamp on IPD Cayman S plenum and slide into place in between manifold hose clamps. Note that the hose will likely need to be trimmed to accommodate the smaller diameter brass nipple on the IPD plenum.

Trim vent line at red line about one inch back from the end
Install vent line to the brass nipple on the underside of the IPD plenum with the squeeze clamp

17. Slide rubber boots onto IPD plenum. Be VERY sure that rubber boots are properly seated,aligned and positioned to as to avoid any leaks which could cause Check Engine Lights.

18. Attach hose clamps that connect rubber boot to IPD plenum FIRST, leaving the manifold clamps still loose so as to be able to maneuver plenum up and down for proper orientation.

19. Angle plenum upward so you can visually verify that the rubber boots are properly spaced by looking down inside the IPD plenum.

20. Snug rubber boots that connect to manifold tight enough so you can still adjust the angle of the plenum manually.

IPD plenum installed between manifold boots with clamps snug, but loose enough to move plenum into position

21. Place factory hose clamp (previously used to attach original factory ribbed hoses to Mass Airflow Sensor) onto IPD silicone hose and push silicone hose onto MAF sensor housing.

22. Now for the most difficult part of the install. Place other hose clamp loose on throttle body end of silicone hose and gently drop throttle body down to meet the silicone hose. This might require a large pick tool. Bring silicone hose and throttle body together until they are engaged. Once engaged, lower the Plenum and TB until you achieve the optimum fit that should have the throttle body going “straight” into the silicone hose which should enable the hose to be as “open” (less kinked) as possible. Again, with the engine hoist bracket removed and the crank case breather line free to be pushed back below the Plenum and out of the way you should have ample room to drop the Plenum, TB and Silicone Hose into this desired position. I found that pushing the manifold to the driver’s side allowed the entire assembly to be rotated down far enough to prevent contact with the body. BE CAREFUL not to push down too hard because you will likely crack the brittle vent line underneath!

23. With a pair of pliers (and some rubber in the jaws) bend the tab (on the body) that would normally cause interference with the new silicone hose. This will prevent any unwanted slicing and/or rubbing of the hose causing premature failure. This will NOT cause any sealing issues with the engine cover.

24. Double check to make sure that all connection between the silicone hose and rubber boots between plenum and manifold are properly spaced and positioned correctly, once verified tighten all connections.

IPD plenum with intake hose installed. Note tab bent up to prevent slicing into the hose!

25. Reinstall breather vent line assembly. Reconnect hose lines to the oil separator and the right-hand side of the IPD plenum. The hose that extends to the driver’s side receives a block off cap which is included with kit. The block off cap should snap into place with enough pressure.

Install block off cap on left side hose connection and reinstall vent assembly

26. Double check to confirm that all hose connections, boots, clamps and bolts are tight and secure

27. Reinstall aluminum engine cover and secure with five Torx 30 bolts.

28. Reinstall rubber backed carpet engine cover by inserting the rear tabs into the body slots first. Then flex the cover piece to fit side and then front tabs until completely secure. Be sure that the luggage D rings are in the up position, so they don’t get caught beneath cover.

It will take approximately 5 to 50 miles for the DME/ECU to recognize and fully adapt to the increased air flow and make the necessary adaptation corrections complete. The DME tends to adapt quicker when driven in a more spirited manor. Enjoy your new IPD plenum!