Sponsored by HUMAN Speakers CIS-E Troubleshooting RETURN

This file is intended to aid in the diagnostics, repair, and maintenance of cars with Bosch CIS engine management systems, and also includes a few items not directly related to this system.

  • CIS: 1979 - 1983. Identified by warm up regulator attached to two fuel lines on port side of block.
  • CIS-E: 1984 - 1987. Covered most intensively here.
  • CIS-E3: 1987 - 1992. Similar to CIS-E, with addition of knock sensing ignition system.

Note that if one part contributing to your engine management system is bad, it will often cause other components to function incorrectly and perhaps start to fail sooner. Your car will not perform properly if something is wrong. Waiting to fix a problem because the car still runs is taking advantage of some quality engineering in the short run to damage it in the long run.

This list of links jumps you down the page to the indicated item.
Links in the item section "pop up" windows with pictures of them.

This file was a lot of work I didn't need to do!
If it helped you, you should send $5 or so to me one way or another!

Listed in the order of ease of checking and likelihood of failure:

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"spare" fuse

Located in a little strip next to the rest of the fuses in your fusebox, this 10 A fuse is often mistaken for a spare and removed. Your car will run rough, if it runs at all, with this fuse removed. Cold starting will be almost impossible.

(KE3: the fuse(s) are more likely to be clearly identified and thus less likely to be accidentally removed. Also, the 0 mA idle current of the CPR provides a reasonable "limp home" mode should power to the system be interupted)

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ground at intake manifold

There are two brown wires attached to a stud on the intake manifold - the connection should be clean and free from corrosion, like all electrical connections.

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Idle Stabiliser Valve:
mounting location
off engine

The ISV is used by the CIS-E (and CIS-E3) computer to regulate the idle speed of the engine, by allowing a prescribed amount of air to bypass the closed throttle. It operates when the idle switch is activated, indicating to the computer that the throttle is closed. The computer then sends a varying cycle on/off signal to this device, the percentage of time it is on versus off ("duty cycle") determining how much air it allows into the engine.

The typical ISV problem manifests itself as an idle at the wrong rpm, and is symptomatic of the insides of the ISV being gunked up and sticking open or shut instead of doing what the computer tells it to do.

Removing it from the car and carefully cleaning it with brake cleaner, followed by a light lube such as WD-40, will often fix this problem.

The ISV also will not function properly if the intake manifold ground is poor, since its electrical circuit depends on them.

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Vacuum Leaks

There are a half dozen molded rubber connectors and several sections of hose under the hood of your car, mostly connected to the intake system. These serve various purposes, including vacuum sources for brake assist and distributor advance, emissions control (positive crankcase ventilation), and idle air control. Rubber ages in the harsh environment of an engine compartment, and when it does it can crack or otherwise develop leaks. These leaks will mess up the way your engine runs, or worse yet, will have been previously "compensated" for with ill-advised adjustments, making it even harder to get your engine to run right.

There are a handful of "sandbox" (oops, I mean shade-tree mechanic) methods for locating vacuum leaks, but this is the best thing to do:

One fine summer day (or series of summer days), mosey on over to the auto parts store and buy several feet of the typically two sized hoses under your hood. Remove the old, frayed, worn out hose pieces ONE AT A TIME and replace them with freshly cut bits of your new hose. Do them all.

If it is still light out (and why shouldn't it be?), you can also carefully remove the various molded rubber hose-like bits (some have three or four connections), noting how they are installed, and clean them and inspect them for damage. New ones can still be obtained from the dealer or some of the better aftermarket vendors. Do not "fix" them with duct tape, except to drive until your new parts arrive. Use new parts. Fix it right.

If you found and repaired any significant leaks of this type, you will need to check, and perhaps adjust, your static fuel mixture - it may have been "adjusted" to compensate for the air leaks in the past. This is covered in these files for CIS, CIS-E and CIS-E3 engines.

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control pressure actuator current, cold and warm

The CPA current determines the fuel pressure used as a hydraulic fluid to set the fuel flow in the fuel distributor. When the engine is cold, it will typically be on the order of 50 mA. At warm idle it is supposed to be 10 mA (it will fluctuate up and down a few mA with the oxygen sensor cycle).

(CIS-E3, with the darker grey CPA, has the advantage of idling at 0 mA, allowing a better "limp home" mode.)

Measuring this current is shown better than I can here at this file.

A warning: if you adjust the mixture to optimise this setting while your car is running poorly, you are probably compensating for another problem. Fixing that problem will then make the car run poorly again until you readjust the mixture. It is unlikely that the static mixture is a primary problem with a poorly running car unless you suspect it has been tampered with.

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control pressure actuator clogging, function

The CPA has a small electromagnet inside it that operates a valve that controls the amount of fuel that flows through it, to regulate the engine fuel supply (relative to the air sensor plate position) at the fuel distributor. I know this sounds like an unholy complication, but it is surprisingly simple and trouble free in practice. If you remove the CPA from the fuel distributor body, you will see two small fuel ports on its "working side", with little O-rings sealing them. There are tiny screens to keep it clean and if they get clogged with junk the device will not work properly. Cleaning them can't hurt much...

If your mechanic has told you you need a new fuel distributor you might want to make sure it not just a clogged or faulty CPA, since they are cheaper and easier to replace. A known good unit from another vehicle or parts stash can be substituted to determine if this is the case.

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Idle and wide open throttle switches

CIS-E uses two switches to enter "special" modes of operation. One of these two switches tells the computer when your foot is off the loud pedal. Under these conditions the engine is run at settings that are especially efficient, or at least very clean. At idle, the ISV is used to regulate the amount of air reaching the engine, to maintain a steady rpm. The other switch is activated during the 30% or so of widest throttle opening, and when the ECU receives this signal it abandons economy and cleanliness in favor of running the engine at a slightly rich setting. This puts the engine into a more powerful fuel/air mixture range, and also helps prevent knocking.

(In between these points, it runs in "closed loop," reading the oxygen sensor on the fly and continuously adjusting the fuel supply to minimise emissions and maximise fuel economy.)

To test these switches, just unplug the connector(s) going to them and use a multimeter set to "ohms" or "resistance" to measure what they are doing. When the throttle is partly open they should both read "infinite" or "out of range". This means the switch is "open." At a rest throttle position the idle switch, which is usually under the throttle body, should read "zero" ohms. When the throttle is opened most of the way the WOT switch, which is usually visible on the top of the throttle body, should read "zero" ohms.

If either switch does not function properly, cleaning may help, but the best thing to do is replace them.

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Fuel Injector Seals

The fuel injectors are at the end of the four or five fuel lines that go into the engine, and are pushed into the side of the head (or in some cases, the intake manofold). This "push in" fit is accomplished by the fuel injector seals, which are thick rubber o-rings halfway up the injector body. By jamming into a corresponding groove in the head they hold the injector in place and also seal the intake tract from air leaks. They get hard, deformed and even cracked with age, and are fairly easy to replace as shown in this file.

The one shown here is an early style, with a single seal. The later injector systems hava smaller seal at the tip, as well. This is due to the "air shrouding" system used to cool the injector tips and aid with hot starting.

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oxygen sensor

The Oxygen Sensor is used by the Electronic Control Unit as soon as it is warm enough to fine tune the air/fuel mixture (via the control pressure actuator). On some cars (90 Quattro, 4000 Quattro), it is mounted in the front of the catalytic converter; on others it is mounted closer to the engine, in the exhaust manifold. The ones located further from immediate engine heat usually have a built in heater to get them up to operating temperature faster (the OXS has to be at something like 600 degrees to work properly).

To check to see if the OXS is functioning, locate the connection in the engine compartment. The one shown is on a car with a heated sensor. These have two separate plugs, one for the signal and one for the heater. Cars without heated sensors (and I suppose some with them) will have the connection in other places, such as near the right side strut tower in the case of a Coupe GT. You can pull back the boot on the signal connection and expose the metal tabs. Leave it plugged in. With the engine warm, connect a multimeter set on a fairly low "volts" range (between 1 and 12) with one lead to the these tabs and the other to a good ground. You should see a steadily fluctuating voltage, that swings up and down between about 0.2 volts and 0.7 volts (200 mV - 700 mV).

If it hangs at either end of this range, either the OXS is not working, or something else is causing a too-rich or too-lean condition beyond the systems ability to compensate. Potential culprits are vacuum leaks, a bad temperature sender, horribly uneven fuel injectors or badly leaking seals, a badly adjusted static idle mixture, or air leaking into the exhaust upstream of the OXS. In other words, almost anything...

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catalytic converter breakup
Typical catalysts
A view of the honeycomb

The catalytic converter is usually located just behind the engine compartment, as the first component of the exhaust system that is "flat" under the car. They are tin cans bigger than the exhaust pipe, with a honeycomb-like element inside composed of some weird metal (a platinum alloy?) that catalyses a pollution-reducing chemicla reaction in the exhaust gas as it passes through.

If the honeycomb physically breaks down, parts of it can come loose and get stuck in the tin can or the pipes and muffler behind it, blocking the flow of exhaust gas. This is often experienced as an inability to generate much power, or rev up the engine. The crude check for it is to rap the tin can with a hammer and see if it rattles. If it rattles, it is coming off for replacement! If not, you will want to determine other possible causes for your problems before removing it to inspect it.

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temperature sender for electronic control unit
off car
on car

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fuel pump relay

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fuel pump

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electronic control unit

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air cleaner

"CIS" air cleaners are a pain to change. This is because they are usually the last component before a device that measures the air flowing into the engine by having it raise a light metal plate in a calibrated cone. So the air cleaner usually ends up mounted below all the air measurement, fuel flow control and fuel distribution junk.

The air cleaner should be replaced about every 20,000 miles, more often in harsh or dusty service.

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spark plugs, cap, rotor and wires

These all go bad slowly. The key to "tuning up" modern fuel injected, electronically ignited automobile engines is to remove parts that still work and replace them with new ones, ideally when the old ones are about 95% wasted. We use service intervals to help us guess when this will be.

The cap and rotor should be replaced annually or every 15-20,000 miles, or if they show a lot of nasty black burn marks at their contacts.

There are many expensive or trick plugs out there. It is more important to have them not be worn out or poorly gapped! Your basic spark plug, at about a dollar or so each, seems to last me about a year, or 15,000 miles. The four dollar Bosch platinums, which I really like, last a lot longer, typically up to 30-45,000 miles.

The OEM (typically Bosch) park plug wires for these cars last a long time, but do eventually need replacment. Signs of poor wires are irregular, rough running (or "missing") when it is very humid out, visible arcing (sparks) or cracking of the rubber covers. To test wires that look ok, you measure their resistance with a multimeter set on "ohms." Your manual should be used to confirm this, but typically the spark plug wires should measure about 6,000 (6k) ohms from the cap end connector to the spark plug end connector, which is way up inside the metal boot cover. The coil to distributor cap wire should be about 2,000 (2k) ohms.

I replace my cap and rotor every year, my plugs every other year, and wires almost never.

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oil and filter

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thermostat and coolant

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coil specs

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injector leakage

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fuel pump check valve

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thermo time switch

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Cold Start Valve

This is an often maligned, very rarely bad part of your fuel system. It is always the first suspect in poor cold starting situations, due mostly to its name. What does it actually do? For a few seconds when you first turn the key (depending on how cold it is under the hood), fuel is sprayed through this "sixth" injector (well, we call it that in the five cylinder engine) into the intake manifold. This makes the mixture nice and rich for a moment, which helps get those first few sparks to make explosions.

There are fairly simple test procedures outlined for them in the Bentley shop manuals, which I am not going to duplicate here. if your car is not starting well in the cold, 99% of the time it won't be the CSV itself. Once in a while the sytem that triggers it will fail to operate properly, but the problem will usually be elsewhere even in that case.

The CSV does so little... it is an electrical injector that has constant power (12v). When the engine is cold it is grounded while cranking through the thermo time switch, and of course sprays a little extra fuel. On some cars, it is also grounded intermittently while cranking by the hot start pulse relay, adding a bit of extra fuel during warm starts.

It can be triggered manually also.

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electrical connections
(Battery terminal corrosion)

Folks, this is the mantra (repeat after me):

Undo every electrical connector or connection under the hood, clean it carefully, and reassemble it using dielectric grease to protect it from moisture!

Your engine will be easier to clean, run better in the wet, and be easier to trouble shoot when something goes bad if you are sure that at least all those little electrical signals are getting to and from the various components properly!

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scuzz and filth reduction

Many components of your engine and its management systems will function better and last longer if they kept clean (cam cover polisher!). The oils and vapors present under the hood, to say nothing of various road salts, dirt, acid rain, and new-cu-lar waste, are all abrasive or caustic in one way or another. Keeping their build up to a minimum will do several things for you and your engine:

  • Prevent or delay disintegration of various rubber parts.
  • Allow parts to heat up or cool off as they were intended to by design.
  • Make it easier to see the condition of parts.
  • Make it easier to see location of leaks and damage.
  • Slightly minimize the grime factor when you work on it.