Diagnosis - Engine

Rough Idle and Engine Shaking - the Ultimate Guide!

Before checking 1-25, try the Shogun RESET Procedure here

(Including information about Catalytic Converters)
Thanks to Johan for examples/pictures on his website –
1) Spark plugs, plug wires, rotor & ignition coils - Clean and inspect.
2) Check compression – poor cylinder head compression is common for an older car. Big job to fix this.
3) Use a good fuel-injector cleaning treatment into the tank – take it for a good run.
4) Change fuel filter.
5) Remove and clean injectors – might need to replace injectors.
6) Hose to fuel pressure regulator – might be cracked/worn (buy a new one)
7) Rubber hose between Airflow Meter and throttle – cracked?
8) Mass Airflow meter/sensor – if dirty clean it up. Bad Air Flow Meter will cause flat spots in acceleration or erratic spots in the throttle depending on the opening of the airflow meter not usually a complete lack of power.
9) Hose to air filter – cracked?
10) Air filter – replace if dirty, check seals.
11) Distributor cap – if eroded and worn - changed to new one< br>12) Check fuel pressure - Replace the Fuel Pressure Regulator. Might need to replace Fuel Pump.
13) Check and Adjust valves - throttle valves, PCV valve behind intake runner, Idle Control Valve (ICV).
• ICV Idle Control Valve. Remove and clean it with brake parts cleaner. This regulates the air coming in to your intake manifold when the throttle body is closed and it tends to get dirty, sticky and the motor inside could not turn the valve properly.
• Idle control valve sometime sticks open – causes issues when in Idle
14) Throttle Body - Clean it out and replace the gasket (can use gasket material cut to size).
15) Check all Vacuum lines.
16) Conduct a ‘leak down test’ on all exhaust valves.
17) Check for Oxygen (O2) sensor function tests.
18) Check the wide-open throttle switch under the gas pedal.
19) Check throttle position switch on the throttle body.
20) Make sure connections on battery (under back seat) are very clean. Use wire brush.
21) Check rubber boot between the Mass Air Flow and the DKM.
22) Check for intake manifold gasket leak
23) Have the transmission checked for correct functioning – (by mechanic).
24) Make sure there are no other sources of drag in the driveline.
25) Check coolant sensor.

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Rough Idle -- Air leak through a fuel injector O-ring

After checking all the rubber hoses for leaks, and the intake gaskets etc. etc. you may need to check the O-rings on the fuel injectors. This shows the top of the injector. The solution is to replace the o-ring.

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No power, but not limp home

03/1988 750iL E32 from Shoreline
I started the car yesterday to clean the garage, as I was putting it away I noticed the EML light did not come on for two seconds like it would normally do, also temp guage does not work nor can I output obc data to cluster, although the oil light etc stil work.

Anyway, I drove it up and down the road and it would not go over about 10kmh, I have fitted new battery recently and the drivers door lock failed, after I tried to resolder the connectors as it had the lock/unlock problem.

Car idles slightly rough and when you pull the MAF connectors, both banks stumble then run as normal so it is not on 6 cylinders. <<Not limp mode then>>

I am charging the battery as the OBC gave 12.09v at key in ignition not started, and 13.5v at idle. 

Tested the eml lights on the console and they are fine, I will look at the X20 and 21 connectors << connections in X20/21 can be become loose >> , it is strange that the obd display in the console does not light up as well as the eml, I will look for any fuses on the way to/from there as well.

(NEXT DAY) found problem Fuse number 20 under the bonnet where the lamp module sits was the culprit, now all the lights come on and the drivers door locks ;-)
I love it when a 15c fuse can render a car unusable.

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Power only at full throttle

1991 B12 from Hairywithit
To replace the rear sub-frame on a friend's B12 we had to drop the driveshaft which meant lowering the back of the gearbox, so we had to drop the exhausts and disconnect the lambda sensors.

With the pressure to get the car running quickly the lambda sensors were crossed on reassembly.
This is very easy to do as the two plugs sit next to each other.
Do yourself a favor and at the next chance take two color paint pens and mark the plugs to prevent this from happening.


Engine starts wthout problem but idle is very erratic -- does not stall but feels like it will
Very little power up to 2000 rpm but this is not limphome! All gears operate.
Above 2000 rpm with big throttle opening the engine returns to full power.
Fuel consumption was terrible.
No warning lights shown -- this car has the Japan-specific high temperature warning sensor for the catalysts and this warning light came on a few times.

Visual Check: The normal condition is for the right side bankconnector (#1-6) is towards the car front, while the one for the left side bank is towards the rear of the car.
If they are crossed the wire on the right side will hang down more.

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Engine will not start and the OBC display is off

If your OBC display is dead and the engine does not start, it could be the fuse for the OBC. Check also the plug into the OBC!
That fuse is hidden because the OBC holds the code responsible for controlling the anti-theft device.
Lift the rear seat behind driver and you will see a hole on the seat frame. You should be able to remove the fuse with the fuse tool from the fuse box thru that hole or you have to remove some of the modules and maybe some screws where the other modules sit.
The module is fixed to the base frame of the rear seat drivers side with 2 screws, accessible from the legroom after removing the carpet.
This shows the cut-away in the LHS rear seat frame.

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Engine overheats

There are many reasons for overheating so this entry will keep expanding but this is a good first step.

1) refill with coolant.
2) run engine until warm -- but not hot.
3) remove radiator cap.
4) check that there is a stream of coolant coming from the the small diameter line that runs to the reservoir on the firewall.
5) if there is no stream of coolant, the line may be blocked or, in some cases, the outlet of the reservoir may be blocked.

Here is the original explanation with a great photo at the bottom of the page!

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