eumecon

Hi, I have a '88 GMC G2500 Vandura with a completely stock 305 TBI. Recent purchase as a "project" (very cheap but with numberous problems to be addressed...)

The engine seemed down on power as noted by the prev. owner, plus the there was leaking coolant from the intake manifold at the rear. I replaced the gaskets (discovered they had been placed with the blocking ports at the front), and took the rocker covers off and adjusted all the valve lash. Started her up and something didn't seem right. Checked with a drop test and saw cylinder #5 had a dead miss. Since then I have done the following:

- checked for spark at distributor cap #5 - jumps 1/2" to test ground wire
- checked for spark at plug, also swapped the 5 and 7 plugs and wires
- double checked firing order - in any case I would assume if it was wrong then I would have more than one cylinder misfiring
- checked with vacuum gauge connected to the brake booster inlet, there was a fast fluctuation just over 20 inHg - confirms there is a cylinder miss, otherwise engine is ok.
- checked compression on #5 - 160-180 psi
- removed rocker cover and checked valve timing/movement. valves were opening at the right times compared to rotor arm positions
- re-adjusted valve lash and double checked using a leak down tester. I originally adjusted the valves using 0.0015" feeler gauge then 3/4 turn, but noticed when I re-adjusted the valve and checked with the leak down tester anything more than 1/2 a turn and I got leakage so left it there.
- checked TBI pulse on both injectors with scope - seemed pretty unlikely it would only cause a single cylinder misfire but running out of options!

Needless to say, everything checks out as far as I can see. I really wish I had checked for misfires or other problems before tearing into replacing the manifold gaskets but here's where we are now.

My only guess now is I have caused a blocked port or serious leak on the intake of cylinder #5 somehow through changing the gaskets. I would have thought that this would show on the vacuum gauge though? Just wondered if anyone could think of some bright ideas for things I might have otherwise missed before I have to take the intake manifold off again. Thanks in advance.

kevinkpk

When you swapped 5&7 how do you know the misfire didn't swap also? If the bolts where ok installed into the manifold, I'd not think the gasket was overlapped into the intake port, just a thought.

kevinkpk

Also with this post, check the condition of the spark plug(s)

eumecon

Ah yes ok, after I swapped the plugs and wires, I drop tested it again and #5 was still missing, #7 still fine

eumecon

Good question - the #5 plug was a little black and wet, but I've been running the engine only for a few minutes at a time while working on it so it could explain it. #7 didn't look much different. I'll pull another plug though to compare it and post some pics.

kevinkpk

Plugs shouldn't be wet even with a little run time, still thinking.

cerrem

When you run the engine, do you smell un-burnt fuel out the exhaust ???
Does the miss occur at all RPM's ??
You 100% sure the #5 valves are properly moving...that you confirmed twice I believe..
Good compression ...OK
This is where I really like to hook up my Scope and look at the coil charge, and the spark line..see attached photo I took..
That could be usefull tool..
Back in the 80's I had a 305 that had similar issue...driving my crazy till I threw tools across the shop....
I finally figured it out..... The press-in rocker stud lifted up from the head...on the intake valve....just enough to make it look like it was moving but not quite enough to open the valve...

eumecon

Thanks for the answers - glad I am in the company of others have been driven crazy at such issues!

Took me a while to reply as I spend a couple of weekend going over everything again:
- Swapped ignition leads and plug between cyl 5 and 8.
- Double checked spark jump from distributor
- Did compression test again and leakdown test, leakdown is barely even 10%
- Removed intake (again) double checked for blockages, replaced intake gaskets
- Removed rocker cover, checked valve timing and measured valve movement - cyl 5 seems to match exactly cyl 7 (4.2 mm if I recall) - not sure if this would rule out an issue with rocker studs etc?

I have been checking for a misfire at idle with a drop test by inerting a paper clip between the ignition lead and distributor and grounding this. Will try this again at higher rpm tomorrow.
I have a cheap hantek scope which I have used for a few things in the past but not coil charge - I'll see if I can figure out how to connect it tomorrow as well

eumecon

I spent some time learning about ignition waveform analysis and bought a couple of capacitive pickups for the scope. Managed to capture the mifire both and idle and higher rpm. I put a pickup (CH1) on the king lead to see the waveform for all cylinders, and then CH2 onthe number 5 lead. Ok so the first picture is the waveform of #5 at idle (center) with 6 and 7 either side.



You can see both the firing and spark line are both lower voltage that the other clyinders. duration seemed ok. A couple of other observations.
1. The firing ans spark voltage seem both very low. The hantek pickup probe literature stated an effective 1000x attenuation so I have the scope set to this and it hits barely 1kV which I am sure is way too low but that is the case for all cylinders
2. There is a high tail for all cylinders which I belive indicates a lean condition. Turns out my fuel lines are junk and partly collapsed, so I will try changing them next.


It gets much more interesting though at high idle, it looks like there is an oscillation in the firing voltages with both 5 and 7 looking about equal



Could a worn distributor cause this? I'm not sure what else would. Mine seems to have about 6mm or 1/4 inch of play but from what I understand this is not unusual.

kevinkpk

Certainly could be, note the amplitude difference on #5, also not consisent but much closer than the #5. The 1/4" is this the shaft play in the distributor? That seems high to me. As suggested check the valve rocker stud, I'd suggest take #5 to full open (both valves, two checks) with your compression gauge, should be 0 on both strokes, that to me, would satisfy valves are doing what they should. I would think if the excess play in the distributor would cause issues across the whole engine, but I had an 82 with ESC, any little thing with that truck would cause grief trying to figure it out.