Engine specs please!

[King Fisher]

Hi chaps, Nick has noticed his car is running oddly, idling too high, and the car smells very rich. So he and his college tutors dragged out and old-school Snap On machine designed around 80s engines. The machine hooks onto the battery, coil, king lead, and number 1 cylinder lead. It then measures the idling RPM (which was all over from 790 up to around 850), spark plug voltage, and also what effect each cylinder has on the engine's RPM (by dropping each cylinder out one by one.

Firstly, the spark plugs had roughly 2kV going to them; it should be much higher according to the manual.

Secondly, when the machine dropped each cylinder out, the engine SPED UP. They did the test again, and cylinder one dropped engine RPM by 97RPM, but cylinder 4 only dropped it by 4RPM?

The tutor's hypothesis is that the spark plugs may be gapped incorrectly (Nick used the internet to find the correct spark plug gaps, but got differing information depending on whether the engine was in a Granada or Sierra!). Has anyone got the 100% correct gap measurements?

[phillpot]

King Fisher said:

the spark plugs had roughly 2kV going to them

Has anyone got the 100% correct gap measurements?

Should be more like 20kV

flat seat plugs (with a sealing washer) 0.7mm
taper seat (catalyst engines?) 1.0mm

Unless they are a mile out they shouldn't, I wouldn't have thought, be causing his problems.Has he knocked one putting it in and closed the gap up?
Did they swop plugs 1 & 4 to see if issue stayed on cylinder 4 or moved with the plug?

In the old days we set them to the thickness of a thumb nail, never a problem

[GreenV8S]

I don't enough details of that engine management system to trouble-shoot it, but if the plugs aren't getting enough voltage they could have an intermittent or weak spark which could cause it to missfire, hence varying idle and smell of fuel. It's the sort of thing that could come and go depending on the state and temperature of the plugs. Usually, you'd expect the symptoms from a weak spark to get better when the engine had just been heavily loaded and was now idling (hot plugs and low in-cylinder pressure) and worse under load on a cold engine (cold plugs and high in-cylinder pressure).

[King Fisher]

ok, could it be distributor based? He had the engine dyno'd and it produced an impressive 172bhp, but has now fallen out of tune. All he's done since having it rolling roaded was take the injectors out and have them ultrasonically cleaned; surely that won't knock the entire engine out of tune?

Also, will it damage the car to drive it in this state?

[GreenV8S]

I suppose it could be, anything that weakens the spark could involve the dizzy. Is the central brush still present and contacting the rotor arm OK? No excessive burning or pitting of the rotor arm tip or contact posts? I heard of one engine with the dizzy base plate in the wrong position so that the rotor arm was almost missing the corresponding post and sometimes cross-firing to the next post, and that caused a weak and erratic spark. To see whether that's the problem you could see what sort of spark you get on the king lead and then compare that with the spark at the plug.

[King Fisher]

GreenV8S said:

I suppose it could be, anything that weakens the spark could involve the dizzy. Is the central brush still present and contacting the rotor arm OK? No excessive burning or pitting of the rotor arm tip or contact posts? I heard of one engine with the dizzy base plate in the wrong position so that the rotor arm was almost missing the corresponding post and sometimes cross-firing to the next post, and that caused a weak and erratic spark. To see whether that's the problem you could see what sort of spark you get on the king lead and then compare that with the spark at the plug.

Ok, how would we do that? Thanks for your advice by the way!

[King Fisher]

GreenV8S said:

I suppose it could be, anything that weakens the spark could involve the dizzy. Is the central brush still present and contacting the rotor arm OK? No excessive burning or pitting of the rotor arm tip or contact posts? I heard of one engine with the dizzy base plate in the wrong position so that the rotor arm was almost missing the corresponding post and sometimes cross-firing to the next post, and that caused a weak and erratic spark. To see whether that's the problem you could see what sort of spark you get on the king lead and then compare that with the spark at the plug.

Ok, how would we do that? Thanks for your advice by the way!

[GreenV8S]

King Fisher said:

Ok, how would we do that? Thanks for your advice by the way!

You need a way to measure the spark strength/voltage. Traditionally you would do this by connecting the HT lead to a spark plug with the body well earthed and position the plug so that you can see the spark. If you want to be scientific about it, you would need something similar to your diagnostic machine, capable of telling you what the spark voltage/strength is.

You do the test with the engine stopped. Apply the normal running voltage to the coil +ve. Usually this is a nominal 12V but for a ballasted coil it would be 9V. It's whatever you get with the ignition on and the engine not started.

Briefly (1/10th of a second) earth the coil -ve. As you disconnect the earth, the coil will spark. There may be a voltage spike on the earth wire when you do this so don't be touching the conductor, but it won't be the high voltage you get on the HT side. By watching the spark across your plug, you can gauge how strong the spark is. Now do the same thing but connecting the plug to whichever lead the rotor arm is pointing to. If it isn't pointing to any of the posts, nudge the engine over until it is.

If the spark is significantly lower when it's going through the dizzy, you know that something is happening in the dizzy that is impeding the spark. For example you might have a track inside the dizzy diverting the spark to earth, or poor electrical contact at the rotor arm.

[King Fisher]

GreenV8S said:

You need a way to measure the spark strength/voltage. Traditionally you would do this by connecting the HT lead to a spark plug with the body well earthed and position the plug so that you can see the spark. If you want to be scientific about it, you would need something similar to your diagnostic machine, capable of telling you what the spark voltage/strength is.

You do the test with the engine stopped. Apply the normal running voltage to the coil +ve. Usually this is a nominal 12V but for a ballasted coil it would be 9V. It's whatever you get with the ignition on and the engine not started.

Briefly (1/10th of a second) earth the coil -ve. As you disconnect the earth, the coil will spark. There may be a voltage spike on the earth wire when you do this so don't be touching the conductor, but it won't be the high voltage you get on the HT side. By watching the spark across your plug, you can gauge how strong the spark is. Now do the same thing but connecting the plug to whichever lead the rotor arm is pointing to. If it isn't pointing to any of the posts, nudge the engine over until it is.

If the spark is significantly lower when it's going through the dizzy, you know that something is happening in the dizzy that is impeding the spark. For example you might have a track inside the dizzy diverting the spark to earth, or poor electrical contact at the rotor arm.

Ok mate, cheers for that. I'll pass the info onto Nick.

[King Fisher]

Turns out it was a faulty new coil! They nicked one off a tutor's sierra that's currently having the sills and chassis welded, and the car put out 18kv, perfect dwell time, and perfect idle. Sorted. Cheers guys.