Car still idles rough/runs rich after 2 months work - I'm at a wall

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #102

As mentioned the MC3334 IC itself has threshold on/off at 1.8V/1.5V.
GM may have added some circuitry in front which modifies the levels, but not likely to be significant.

The mention of 1.2V and 0.9V in the pdf file is another matter. That relates to putting a steady DC on the input of the GM unit which does not happen in normal use. I just found by accident putting a steady DC above 1.2V on the input really heats up things very quickly. Important to me and anybody using the GM unit with an aftermarket EFI system.

I configured the Motec unit to drive the GM unit with a signal between +0.5V and +5V. My bet is any signal from about +0.5V to +3.0V would work safely. Also the Motec unit uses its own logic for dwell control.

In the pdf file I had the GM unit on the bench with coil and spark plug attached. I used a signal generator to drive the input at +0.5V to +10V, and I varied the duty cycle. You can see the on/off of the coil was synchronised with the input pulse waveform. Ideally you always want the coil primary switched on ( its -ve terminal switched to ground ) only long enough to store the energy you need to create a good spark. Keeping it switched on longer just heats the primary winding. Also you want the current to be limited which the MC3334 can do.

I was using a Bosch low impedance coil, nothing special, and got reliable spark up to the equivalent of 12,000RPM. In an engine the spark characteristic could be different and might not get to 12,000RPM.
That 12,000 RPM does show only 0.5mS is needed to charge up the coil.

It also shows the low RPM operation is very wasteful if the coil is switched on as long as the pdf file shows. I would have to do more testing to see how the GM unit works with an actual dizzy signal, and how the dwell time is controlled.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #103

All things considered, the dizzy should be wired for the correct positive polarity of the pulse, even if by luck it can work when the wiring is reversed. So far nobody has reported the new dizzy wire Greg has bought is right or wrong.

To be 100% sure you need to check with an oscilloscope.

(Michael Garcia) #104

…maybe, in conjuction
with an ignition problem,
when l look at the initial symptoms,
google what happens
when the oxygen sensor doesnt work properly…


To turn the engine by hand, I have a specific Craftsman 1/2” ratchet drive that narrows towards the head. It just fits in the gap between the crank pully and the front obstructions when I go in from under the car.

Not too hard, but impossible with all my other drives.

(Greg) #106

Thanks, I’ll give that a try. I was also wondering if I could turn it by using one of the nuts holding on the crank pulley? May be easier?

Question about spark plugs, I am using what was recommended in the book. NGK 2238 TR5, gapped to 0.025". Is this good? I’ve seen several other recommendations. Want to make sure I’m not running too cold a plug. Thanks.

(Michael Garcia) #107

if your exhaust system is
namely the manifolds,
this will reoccur
no matter what you do
with the sensors…

(Greg) #108

Only had two hours to work on car today, here’s what I accomplished:

New ignition amp, I can start it again. (Thanks SNG for fast shipping!) Still difficult start (I’ll blame fouled spark plugs) But the spark is MUCH bigger now, like a full 1 1/2" from coil wire to ground, enough that it went up my thumb (ouch), but still a combo of blue/orange. Spark was no different with polarity reversed on pickup coil.
Car still idles the same, rough. Vacuum still at only 10 in hg. Still revs ok.

When I tried reversing the polarity on pick up coil, I could only get it to run with vacuum advance disconnected and the dist retarded a bit. It revs and idles almost identical. Vacuum is also 10 in hg.

Pulled plug #1 and got it to TDC (thanks to whoever suggested using a straw!) Rotor was pointing about 50% past (advanced) cap post #1. Timing mark was at 0. Harmonic damper seems stable and timing mark plate looks good. Based on this, I believe my timing was good, and that I should NOT reverse polarity on pickup coil, because I’d have to retard the timing so much that at TDC the rotor would be well before cap post #1, which doesn’t sound right.

While idling cold and idling warm, pulling CTS plug did nothing. It was 54f today.

While idling, tripping throttle switch made it idle worse.

Pulling TPS plug while running made no difference during idle or revving. Tested black Bourne TPS, got 0.34V at idle and 4.9V at full throttle. Check.

Tomorrow I am going to pull all spark plugs and do a second compression test, and replace fouled plugs.

Next weekend I will test ignition system based on that slight orange spark (I may try extra engine ground?). And I’ll test exhaust clogging. And a few more things on my list.

Slow but sure.

(Philip Lochner) #109

Are you saying the rotor tip was halfway between #1 and the adjacent terminal? With crank at TDC, that would make timing way out!

If the tip was still overlapping the terminal by 50%, then the situation is better. But then you used the word “advanced” which makes things a bit unclear again. The rotor turns anti - clockwise, therefore, in the above situation, the rotor tip should be a bit more anti-clockwise relative to #1, but still overlap it.

(Greg) #110

Yes, at TDC and pulley at 0, the rotor was about 50% between #1A cap post and the next one (#6B).

In the book, for static timing it says to go 75% past #1 terminal.

(Aristides Balanos) #111

You spark now, so that’s good news Greg !

There is something wrong there, and I believe this is the reason of your troubles…
Either your CTS is not good, either it’s connection to the ECU, either the ECU itself.

Did you try to short the CTS terminals with a paperclip or something to see if it makes a difference ?
Try also to adjust the ECU’s Base Fuel Map.
That’s where I would start.


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #112

That ain’t right. You need either a new CTS or to fix the wiring to it.

(Greg) #113

CTS is new, resistance was perfect. Resistance was identical at ECU prongs. Shorting it out with clip did nothing. ECU?

Regarding timing, Philip got me thinking. Have I misinterpreted the book? When I put distributor in with static timing, I saw in the book an expert recommended 75% past #1A terminal for perfect timing. So at TDC I meshed the distributor gear so the rotor was about 50% past (counter clockwise) terminal #1, halfway between #1A and the next in the firing order, #6B. Was that supposed to be 75% of the rotor width, NOT 75% of the rotation between terminals?

The Book: “The centerline of the rotor will be 75% past the No. 1 line on the inner shield.”

(Philip Lochner) #114

Hi Greg

I’m 100% convinced you have misinterpreted the book. I believe your dissy shaft should be re-meshed 1 tooth clockwise.

75% “past” the #1 terminal, but still overlapping it sounds A LOT MORE PLAUSIBLE than 75% of the distance between 1A and 6B.

I am intending to post a pic from my E-type dissy. I just have to move my Jensen Interceptor off my pit (after refilling the transmission) so I can access the E-type from below, to set it to TDC/1A. Just waiting for the sealer to dry a bit before refilling.

(Philip Lochner) #115

I wonder if that is not a misprint… 7.5º would make so much more sense as that would be 15º BTDC on the crank.

(Greg) #116

So rotor turns 30º between plug terminals (360/12).

I assume that equates to 60 crankshaft º? (crankshaft timing)

(Philip Lochner) #117

Correct on both statements.

(Philip Lochner) #118

Here’s what I have on my E-type:

  1. First I set up the rotor to point dead centre to terminal 1A. Crank came to 20º BTDC.

During my early EFI days I had issues with rotor phasing, hence the hole in an old dissy cap to show how the rotor is pointing exactly to 1A:

Now with dissy cap removed. Note the dent on the top rim of the dissy housing and the one lower down at the timing vernier marks which indicates #1 HT terminal dead centre.

  1. Then I set the crank to TDC and then the dissy looks like this:

Now, some comment on the 20º BTDC: I’m one of those who ignores the manual when it comes to timing specs. My reasons for doing so are not “politically correct” but they are good for my engines. The 20º BTDC results from the engine being 8:1 compression and being used mostly above 4750’ (1450m) AMSL. At sea level, I would probably aim for 15º BTDC.

My car is fitted with Lumenition ignition system - hence the interior of the dissy appears different from OEM V12 dissies.

(Robert King) #119

What has worked for me is that the right side of the rotor needs to be pointing at the 1A tower when crank indicator is at TDC (0 degrees); that allows rotor to be pointing at the tower as the timing advances.
The relationship between the reluctor tooth and the rotor is fixed in the distributor design(unless something is assembled wrong between the reluctor wheel and the upper advance mechanism). Is the nylon stop intact with the pins in the proper place? Is it possible to put the reluctor on upside-down? Does the 15 degree offset of the pickup polarity play into this?

(Greg) #120

I reinstalled distributor with rotor pointing just a couple mm (still overlapping) counterclockwise past 1A at TDC. This looks and sounds correct to me. Car still runs poorly. I tried reversing pick up polarity, and it would barely start or idle. So I’m assuming default polarity is correct, even though the wire colors are reversed. I verified my timing is correct, was 10degrees BTDC at idle with vacuum advance. That’s close enough for me for confirmation.

Star wheel looks symmetrical, so I doubt it’s upside down.

I wasn’t able to do proper exhaust restriction test due to engine not able to rev anymore.

I listened to injectors, they seem to be firing correctly.

Again, the symptoms: I can usually rev it to 1500-2000rpm no problem and it holds and sounds smooth, except for random missing causing entire car to shake. Like a miss every 1/4 sec - 1/2 sec.
I let it idle, it hunts for rpm, and then stabalizes to about 700-800 rpm. Randomly misses/shakes a lot at idle, like it may stall, but doesn’t. Runs very rich at idle, I can smell it. Very very low vacuum at idle and revving.

What I’m concerned with now, last weekend I could get 12 in Hg vacuum at idle. A few days ago, I could only get 10 in Hg. Yesterday it would only idle at 7 inHg. And by end of today testing, it only idles at 5 inHg, and running at it’s worst. It then really bogs down when I try to rev it. Even though I’ve been improving everything, the car seems to be going downhill over the last week. Next weekend I’ll pull plugs and do a second compression test. Hope there’s not something major. :frowning:

If compression is still good (was 180-195 in Oct), exhaust/cat clog could still be at fault? Still thinking of what Michael said. There is a lot of soot coming out of exhaust pipes, a slight exhaust leak before right cat, and now I’m getting soot stains on the ground centered behind the engine where the downpipes go to the cats. And I’ve been running the engine a lot this week for testing. And I swear there is carbon soot in the oil?

PS Philip, thanks for all those photos!

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #121

Interesting chopper wheel! That looks completely different than the slotted wheels we discussed fabbing a while back.