Summary of problem: '87 T. Car has untouched stock motor with basic mods: blue tops, TA49, recored stock IC, ATR stock-type downpipe, mild lockup converter. Car has a bad cut-out around 2200 RPM. However, its not on every engine revolution. More like in 1 second intervals or so. Much more prevalent in gear than in park or neutral. Also happens slightly at another interval of the rev range, but haven't pinned it down yet. Somewhere around 3500 or so. Does not have to be under boost, and in fact seems to happen best when NOT under boost. Sounds like it completely looses ignition, and then comes back. Car has silky-smooth idle, and otherwise runs great. Easy to reproduce by just sitting still with foot on the brake. Just pop it in gear and rev up to 2200 or so.
Have changed out the ignition module. No change. Have swapped out MAF with a known good unit. (Thanks Dwight!) No change. Also happens with MAF disconnected. Verified system voltage. Completely stable with no variance during the behavior. Have changed out the chip. No change.
Next on the list will be new plugs and wires, new O2 sensor, swapping out the ECM, and verifying all grounds.
I realize I haven't thorughly tried everything yet, but I'm a little stumped already. If it was happening on every engine rev, I'd actually feel it was easier to track down. But the way it is behaving makes me hesitant to point to a crank or cam sensor. The motor has a tie-down strap and shouldn't be moving around, especially with the car sitting still. So I don't think I'm yanking on any wires. I don't see any bizzare numbers in the data that give anything obvious away just yet.
Any similar experiences that point to something I'm missing here?
When you change the plugs, do a compression test.
You did this test also while watching a fuel pressure gauge right?
FP is steady. I see a *very* slight needle move - not even a needle's width* when the motor is doing its dance, but I attributed that to the vaccuum change when all this is happening. Should have just yanked the hose from the regulator to confirm my suspicion. I'll check this.
I am noticing a little more of this across the rev range in spots I might not have before. But its very prononunced right here. And again in the mid 3k's.
Haven't got the plugs out to check compression yet. Why do I get the feeling you're thinking flat lobe(s) or lazy/collapsed lifter... Still, its not an every-rev phenomena the way I'd expect a flat lobe to sound. Lifter...perhaps.
Have you tested the coil pack and module? I had a slight miss, I guess you'd call it that, would come and go, usually around the same rpms. I tested my setup and had a bad coil, or had one going out. I put a new one on and it went away. If you want to use my tester just let me know, but I'm sure you already have access to one.
I have a Casper's tester, so I can check. I originally swapped the module without testing because the behavior seemed so much like a previous module with one pair of cylinders not getting fire.
I wouldn't be surprised at a coil being the problem at high RPMs or under boost, but with the RPMs involved and such here, that doesn't seem consistent. Still, I'll give anything a whirl at this point. Trying to see if I can think it through before I get too dirty this time.
Just hazarding a guess but I had an intermittent miss I chased for months. Turned out to be the ECM.
Dave Walker
Here's the latest...
New O2 sensor - no change. Different ECM - no change. Compression test - 145 PSI across all cylinders. Plugs and wires - no change. Tested the coil and module - Saw some slight break-ups in a few spots. Hmmm. As I recall, the module was changed in April and it hadn't had any effect. So I grabbed another coil. The one I had been running is probably only a year or so old, and ohms out OK, but was not changed in the process of chasing this problem. On the tester the one off the shelf looked clean. So I put it on the car. And...things seemed to run clean in the driveway! Hey, maybe that was it!
The car sat for awhile as I cleaned up my tools. Eventually went out to give it a test drive. Right away I could see the problem was there again. Just as bad as ever. So much for the coil solving it. However, this time it went away once the car warmed up. Or so I thought. After 15 miles, it didn't hiccup at all on the road. Coming back home I did a few quick brake-torques sitting still and it did burp a little right around 2500 just once.
*sigh*
Mind you that this 'other' coil is a used unit, so it could be bad or intermittent as well.
At this point I may grab a new coil or borrow a coil/module off a TR that's working well. Or swap mine with one that's working well. Either way, try to narrow down the problem. I'll also revisit grounds, and check the crank and cam sensors. I took a quick look at the crank sensor and didn't see anything obvious, but its worth giving it a good look at this point. Plus since I've never even touched the cam sensor, I'm wary of it.
Easy test - how did you test your wires?
Can you get a buddy to load the motor while you spray water out of a windex bottle (or whatever) on your plug wire boots? It makes it a LOT easier to see arcing on the underside of boots where you normally wouldn't detect it.
Just a thought..and it wont cost ya anything.
Set it on "spray" and get a mist of water around each boot..you'll know if it's spark-related because the water will obviously conduct like a maniac if there's a hairline crack that only surfaces under certain engine loads. ?? just my 2¢
Scott, give me a call if you want to borrow another coil and module. I still can't think of anything you haven't aready done.
Craig
Wow - after running at the track tonite - i think i just picked up scott's problem! The same thing is happening to me!
I Just got back here at 1am - got to get up in 4 hours for work
....but I did look at my runs a bit on directscan to try and pinpoint it.. From my scope-readout in directscan..EVERYTHING is affected...O2, Injectors, MAF, you name it...BUT..the first thing that drops is the TPS...all other values follow.
The TPS seems to be the cause - and i have a TPS-TEC, so It may be shorting/malfunctioning. Try bypassing your TPS-TEC (if applicable) as I will try this tomorrow as well. Something's cutting out almost immediately after a really hard launch for me..then again at around 3500 RPM.
Just my first guess after seeing the evidence on Directscan...TPS is going to be my first order of business tomorrow. We shall see..
Latest update...
This weekend I took the cap off my cam sensor. Inside didn't look very dirty, but I cleaned it out. Also checked the interrupter wheel and it was secure. Put things back together, and started the car. Ran the car through the RPM range, and did some brake torques through the RPM range of concern sitting still.
Problem appears to be gone. Hmmmm.
Haven't really taken the car out yet or run it through any heat cycles, so I don't feel that I've truly determined that he problem is gone yet. Perhaps there was just a loose connection at the cam sensor connector that was causing the original problem. I don't recall ever having this connection open or the cap off the sensor.
I also opened this connection while the car was running and could hear the injectors go into MPFI mode. Running through the RPM range both in and out of gear could not reproduce the problem.
I've also swapped back the MAF and coil I had been running when the problem manifested itself, and the problem has not returned.
To answer Jay's question regarding my plug wires, this problem occurred both with my Red Armstrong wires that were on the car when the problem started, and with brand new 9mm Holley Anhilator wires. The latter ohm out at below 100 ohms each, by the way. Seem to be of pretty impressive construction, and use Packard wire. (i.e. the stuff GM uses) I could do without the red plug wire boots, but they're down at the plugs and really don't stick out too much.
And regarding the TPS TEC, I do not use one. I also do not recall seeing any data being scrambled during these episodes, nor my TPS showing a bad spot.
Next step will be taking the car out and putting some miles on it to test, plus getting a few heat cycles on the electronics. We'll see if the problem is licked...
Scott
glad you found it scott...sounds like that was your prob. Mine was in fact the TPS-TEC..I bypassed it, and everything is fine now. Amazing..i've only had it for about 2 months and it's already bad? I was able to squeeze a bit nore adjustment out of my regular TPS, so I dont need the TPS-TEC anymore. Its much more linear without it, and i can finally get 4.59V without the TPS-TEC and .38V at idle. Works great with just the stock TPS.
I think I got lucky on this one..I hope i never have to deal with the cam sensor..the TPS-TEC bypass took all of about 30 seconds to fix :P
Update on this topic: I've put about 50 miles on the car, plus several heat cycles, and the problem does appear to be gone. Ultimate solution seems to have been centered around cleaning out the 'cap' of the cam sensor and reseating its connector. No other real changes. Wouldn't think this part would be that sensitive, but that's all that was changed.
Scott