Diagnosing a possible misfire

[Timbo]

Going back to what you said about metal shavings on the rotor... My guess would be a distributer problem. If your distributer bearing is bad, and causing the distributer to come in contact with the cap, I believe it could cause metal shavings on the rotor. It would also, theoretically, cause misfires.

 

[Timbo]

Also, yes, you should be able to test your TPS with the key on, truck off like that.

 

[Flyin Ryan]

If only stumbling after warm-up (closed loop), might suspect the o2 sensor.
Also

thank you for that info...that seems to be my issue currently

 

[turbodog]

Sounds like it should be getting enough fuel. Therefore, likely a leak. Any vacuum hose leaking will cause this (after all, the IAC is metering air into the engine, if air is getting in anywhere else, the IAC counts will be low in order to maintain idle speed. Also, could be upper intake-to-lower intake, or lower intake-to head, or throttle plate bushings.

I wouldn't do any more to ignition until you fix whatever is causing IAC to be so low. You're fortunate that your scan tool gave you such a clear-cut clue. Now to systematically troubleshoot based on that clue. I'll look at the troubleshooting tables and check back later.

 

[turbodog]

OK, here's part 1; preliminary checks. best to check everything listed, and assume nothing.
My editorial comments are in [brackets]

ROUGH, UNSTABLE OR INCORRECT IDLE, STALLING

PRELIMINARY CHECKS

SENSORS
CHECK: Oxygen (02) sensor - Inspect sensor for silicon contamination from fuel, or use of
improper sealant (never use sealant on any waterproof connectors). The sensor will
have a white, powdery coating, and will result in a high but false signal voltage (rich
exhaust indication). The ECWPCM will then reduce the amount of fuel delivered to
the engine, causing a severe driveability problem.

[probably not your problem, since it misses during open loop operation. But check BLM values! They could be an important clue.]

CHECK: Throttle Position Sensor (TPS) - If a sticking throttle shaft or binding linkage causes a
high TPS voltage open throttle indication, the ECM/PCM will not control idle. Monitor
TPS voltage. A “Scan” tool and/or voltmeter should read less than 1.35 volts with
throttle closed.

[definitely check this]

CHECK: Coolant Temperature Sensor (CTS) - Using a “Scan” tool, compare coolant temperature
with ambient temperature on a cold engine.
- If coolant temperature reads 5 degrees greater than or less than ambient air
temperature. Check for high resistance in coolant sensor circuit or sensor itself.

[already done]

CHECK: MAP sensor response and accuracy

[do you have a mityvac? Makes checking MAP easy. ]

FUEL SYSTEM
CHECK: To determine if the condition is caused by a rich or lean system, the vehicle should be
driven at the speed of the complaint. Monitoring block learn will help identify problem.
Lean - Block learn greater than 150.
Rich - Block learn less than 115.

[as stated above.]


CHECK: Evaporative Emission Control System.

[disconnect hose from throttle body. pull vacuum, should flow a little. If flows freely, check hoses and canister]


CHECK: Perform a cylinder compression check.

CHECK: For injector(s) leaking. Check fuel pressure.

[probably not your problem. Does fuel pressure hold steady with ignition off?]

 

[smeagol]

In my experience, a vacuum leak is going to make the idle race, or at least be *stable*.

When i've setup really custom vehicles, I'll set the throttle so that hot, in gear, IAC is about 30-50. Yours seems in that range, despite what manual etc say.

Go *create* a vacuum leak - i bet your conditions change 100% - meaning what you have isn't due to a vacuum leak. That's just what I'm betting on. You may have one, but I don't think the current problems stem from that.

Anyone local with a Syty or TPI car? borrow their IAC... it's worth a shot, not too much work. They can get stuck, dirty, and so forth.

To test the 02 sensor, go unplug yours when it's warming up. It will stay in open loop mode, prolly set a code etc, but at least if it IS the cause, your symptoms will go away.

Also, since you have changed all the components, it's all the more likely that something ignition related IS causing the problem. You have a problem? what has changed, or what have you changed?

My ignition woes in the past showed up like this:

distributor/cap/rotor issues - just crappy everywhere
wire problems - show up under load, when there is higher cylinder pressure
plug/gap problems - show up most under load, or when leaner

 

[turbodog]

Next:
ADDITIONAL CHECKS
CHECK:Vacuum leaks can cause higher than normal idle and low IAC counts.

CHECK:ECM/PCM grounds for clean, tight, and proper routing.

CHECKark/Neutral (P/N) Switch operation.

CHECK:Use “Scan” tool to determine if ECM is receiving A/C request signal. Whenever A/C is selected.

CHECK:EGR “ON,” while idling, will cause roughness, stalling and hard starting.

CHECK:Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
IAC valve will not move if system voltage is below 9 or greater than 16 volts.

CHECK:A/C refrigerant pressure too high or faulty high pressure switch.

CHECKCV valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.

 

[turbodog]

In my experience, a vacuum leak is going to make the idle race

This is true, but only if the leak is severe enough to drive IAC counts to zero. A lesser leak will drive IAC counts low; ECU will still be able to control idle speed via IAC - but badly, because it is only controlling PART of the air getting into the engine. Result is rough idle, missing, stalling.

Clearly could be many other things, but best to systematically check everything and overlook/assume nothing. I've seen many folks jump around from idea to idea, and the problem ends up being something they 'knew' was OK, and should have been checked early in the diagnosis process.

You really should buy a timing lite, also. The metal shavings should likewise not be dismissed without at least checking for stable timing at idle.

 

[AlaskaTy]

I'm going to run dm again after I get my ignition put back together. The data I was giving you was from early april. When I look at my last run (early may) it shows that when I started my truck the engine temp was 135* (IAT was 96*) it was out there baking in the sun. My IAC values started out at 60 but then dropped to 30 after a couple minutes and then they hung out between high teens and low thirties for the duration. My Fuel pressure doesn't bleed off after shutdown for MANY hours, I have a new (2-bar) MAP sensor, and my compression is 130-140 for each cylinder. I'm going to put it back together without my MSD box, and see what happens. And I'll check the tps, IAC, cannister, visual inspection of O2, but how do I check for vac leaks? I think I remember a bike tube trick but I thought that was only for the IC system. Thanks.

 

[turbodog]

The tire tube trick can be used in your case:
1. Loosen/remove IC mounts and shift it out of the way
2. clamp tube over throttle body
3. pressurize
4. listen for leaks

Note: you need a reasonably high volume air source, since one cyl will likely be in valve overlap. You can minimize air requirement by plugging exhaust pipe (you then get your exhaust system leak-checked for FREE!)

I know a guy who did this, AND he had a small smoke bomb handy. So, he lit the smoke bomb and 'clamped' it in the TB using the throttle blades to clamp it against the throttle bore. Then he clamped on the tube and pumped away. Smoke appeared at his trouble spot. Too cool.

 

[AlaskaTy]

Got the tube, gonna be testing it here shortly, just went and rechecked my rotor (metal shards) and the metal came from the bottom of the coil wire electrode (the one that connects to the center of the rotor. The new one has a nice brass/copper indented swirl and the old one has the swirl but the indentation is a small divot now.

 

[AlaskaTy]

Is there any reason why the dist. cap is rotated in Lee's "syty firing order" diagram?
http://www.whitetyphoon.com/firing_order.html

 

[AlaskaTy]

New cap and rotor, new plugs (NGK UR6), taylor wires, and unplugged MSD box and the idle is way better. It still stumbles and tried to die once, but it doesn't oscillate nearly as much, or as wide (IE: it used to bounce from almost dying to 1200 RPM, now it'll bounce between 500 and 900 RPM - estimates, haven't ran dm yet.)