erratic timing miss 1992 cadillac 4.9l

I repeated the timing check, now only cyl1 show a 6 degree retard. At fast idle. All other cyl are steady. I put the jumper into the aldl connector and i get the same result. The manual fixed spark adjustment show the computer is oversetting by 2deg over the 10deg manual adjustment.

I put the dwell meter in the injector outputs from the computer>

at idle run up to fast idle cyl2 and 7 4 degrees jump to 6 then down to 4. AllOthers 2 degrees smooth movement up to 8 coordinated with rpm.

i unplugged the computer and the injector fuse which isloates them. Ohms of each from the computer side, all 17 +- .3 Ohms to ground, each infinate (greator then 2meg)

dc clamp meter on each injector while in use. cyl2 & 7 60ma others

20ma (repeatable resolution is 10ma)

Sound like the computer is driving 2 and 7 rich at idle and while driving is lean. Car does drive smooth at a very low throttle, but miss at idle and 1/3.

Going to get a new pcm. (ecm)

Now i still dont understand the timing jiggle. If this is a loose chain then all cyl will show it. If this is a dirty dist-gear then maybie? If this is a broken tooth on pickup stator does that affect only one cyl ?

The erratic timing is why I recommended checking the primary ignition dwell, which in reading thru your post, you haven't done.

I think the results of your measuring injector on time with your dwell meter depends on the impedance of your dwell meter and how well each individual injector driver handles that impedance. But, all things being equal, I'd expect the same results for each cylinder regardless.

Are you saying that you measured each injectors resistance by connecting between the 12 volt feed (fuse) and the ECM connector? (sounds like that -is- what you're doing) if so, see next answer.

If you measure 17 ohms resistance thru the injectors, assuming a feed voltage of 14 volts (give or take) the current thru the injectors should measure somewhere near .8 (eight tenths) amp. This also fits with what is normal and expected to be found on most GM PFI products.

10ma and 60ma aren't even close,, so either you're using an amp-clamp not suited for the low amperage test you're performing or you have extremely high resistance in the injector circuit. You need to check for voltage available

-at- the injectors. You also need to test each injectors resistance right AT the injector, which may mean some disassembly of the engine in order to reach them.

Since you did measure significantly different current in injectors 2 and 7, I'd focus on why that is. If a low current inductive amp probe is not available, maybe you can wire in a DVOM and get a direct current reading that way, perhaps your DVOM has a 'min-max' function? My GM book shows the injectors in the 4.9 as having a resistance of 16.2 ohms which will change accordingly with temperature.

It would on an engine that is not misfiring. Once you introduce a misfire or two, the crankshaft doesn't accelerate/decelerate for those misfiring cylinders the same as it would for the other 6 that -are- firing correctly, under these conditions the timing chain if worn can whip erratically. Quite possible that you are seeing multiple problems compounded.

I don't think the 4.9 is known for wearing the distributor drive gear. The 91 Seville I owned didn't do it.

Broken magnets in the pole piece cause all sorts of wierd problems. All it takes is a visual inspection to verfy.

Sorry yes i will check the dwell. I guess somewhere in the book it will tell me what it should be. (yes the primary)

Is there any point in checking the dwell from the computer or to the computer ?

I would think the injector dwell measurement is clean , the computer is switching to ground for a pulse, then the injector is pulling up to

12 at rest. I guess there is some spikes because of the coil in the injector. I dont know the absolute accuracy to a number but i think it is really showing (a relative) pulse width. The difference between cyl is important.

If the computer is stuck driving 2 cyl at the same width. then it matches the other observed symptom which is missing a idle, ok a 1/8 thottle, miss at 1/4 thottle and on. If true then this is a really wierd computer failure ?

Also wierd is that the "diagnositc mode injector flow test" passes. In this test the computer is told to fire each injector for .5 seconds, but this is a static engine off test.

The injector was isloated for the ohms measurement.

I agreee that the (this) dc clamp meter wont measure the actual current of the pulse, but it is averaging. And as such shows a crude rendition of pulse current times pulse width. It was easy to try and also shows a difference in the computer firing of cyl2 and 7.

So a difference could be interpreted as incorrect current or incorrect pulse width. (hope for one problem) But the static measurement of the injectors is fine (in ohms) and path to ground is not present (short) that leaves only the pulse. (did i miss something) I think that meter has a peak (max) function ? i forgot to try that. the injectors were removed and checked and all read 17. the injectors were put back in different positions , but the miss did not move.

The book is so thick i could not find the difinitive spec on ohms. Thanks for standard quote.

I wish i had a scope for this problem.

Ill look more carefully at the pole ( stator ) for breakage,

Update

computer was replaced and the miss is gone.

Historical Review:

START Symptoms

1. miss at idle
2. Idle high
3. no miss at 1/8 throttle
4. miss at 1/3 throttle
5. no signal from oxg-sen
6. Computer code of throttle movment for idle too high
7. Computer code of oxg-sen
8. plugs 2 and 7 are dirty; dark not black not wet no oil, all others very clean.
9. Idle surges and uneven
10. time light shows spark at all wires
11. power balance test (using computer) shows 2 and 7 are low in power. (partial miss)

repair Pass1

1. fix two vacuum hose leaks
2. Change plugs and wires and cap and rotor and ignition module in cap. Symptoms
3. miss at idle
4. idle speed steadier and lower, but still missing.
5. Timing light shows spark at all wires
6. base timing off 4 degrees
7. base timing during fixed diag spark mode not stable. Retards at fast idle and jumpy. This only happens on cyl 1 and 8 during throttle tap. Other cyl sparks are very steady.
8. fixed spark mode of computer (diag) commanded settings of 0 to 14 were not showing up accurate, (with timinglight) generally more plus and minus movement observed.
9. power balance test shows full miss on 2 and 7

repair Pass2

1. perform injector flow test. ( no errors)
2. perform injector flow test rail off ( all spray)
3. remove injectors test ohms , mild cleaning. Replace in different positions.
4. perform injector flow test with rail in air, no problems, all spray.

Symptoms

1. no change from above, still missing the same.
2. perform power balence test, still missing on 2 and 7.
3. perform compression test. All are about 185 (+3 ?5) except 2 and 7 which are 196.

Repair Pass3.

1. Attempt to perform some ground checks , some are good and some are marginal, but not repeatable. Book suggests check ground in dist, and at computer.
2. tighten marginal battery connection.

Symtoms

1. No change
2. put dwell meter on injector output from computer. On each cyl at a time. At idle all read 2deg but cyl 2 and 7 which reads 4.
3. Throttle tap, dwell reads coordinated increase of 2 to 6 deg on all cyl except cyl 2 and 7, which show no change. (fixed at 4 deg)
4. pull plug from computer and injector fuse, check leakage to ground on each injector wire none.

Repair pass 4

1. replace computer

Symptoms

1. no more miss.
2. Timing light is steady during fixed spark diag and throttle tap
3. diag commanded spark setting from 0 to 12 is correct with timing light
4. engine a little rough

repair pass 5

1. Replace oxygen sen
2. clear all codes.
3. drive car 2 days

Symptoms

1. idle very smooth.
2. no roughness.
3. no codes. DONE

Is this a story of a bad computer which cause carbon buildup on cyl 2 and 7 ? Does a computer fail with a fixed pulse to one injector ?