[pedal92]

lately i've developed a random "chug" if you will. this occurs random and may be mild to severe. here's what i mean, you may drive 5 miles no probs then all of a sudden the car will buck like you shut the key off for a split second then be fine. sometimes when it does this it will chug continuously for a mile or two then be fine. when this occurs the wide band shows an extremely lean spike, 18.0 or greater, the factory tach needle will drop about 1-2k for a split second, the autometer tach will drop about 3-4 hundred for a split second, when this occurs all other guages, lights (such as instrument panel lights and headlights) remain unaffected. it seems to to this more while cruising, however it will randomly do this while "standing on it".
one other thing while idleing it sometimes happens. when it does the fuel pressure guage will drop 2-4 psi for a split second. i have a load of data logs i will upload as soon as i get to my laptop that has them on it. thanks for the help.

[decipha]

sounds like its dropping power

step 1> replace ecu and fp relay
2> reground ecu, battery, chassis, block


step 3> enjoy

could be a failing pump

[frepea]

sounds like its dropping power

step 1> replace ecu and fp relay
2> reground ecu, battery, chassis, block


step 3> enjoy

could be a failing pump

^^
what he said..I would try just #2 first. I was having a similar problem..cutting out/running like crap,added a bunch of new grounds,battery to block,to body,to ecu,so far so good.

[pedal92]

decipha i tried switching the WOT cut out relay (i can't seem to find a fp relay close to where i live and the wot cut relay and fp relay were the same) with the fp relay > same thing. next i'll try checking and adding grounds (thx frepea). i have 3 ecms and its pretty much the same thing with all of them. oh and for the first 5-6 miles you drive it its ok then once everything has heat in it thats when it starts. seems i had a 93 gt do this also but i never tried to figure that one out, i sold it first.

[pedal92]

i've fought with this since august. i've changed everything on the car 2-3 times, bought an eec-iv breakout box and tested voltage and wires. here's the tale tale of whats going on. oh and if there are any experts of the eec-iv system on here or i anyone knows of any that i can contact elswhere let me know.

here it is, the car random chugs at idle now. i can connect a timing light to the coil wire and as the car misses the timing light also misses. so an ignition misfire.
back probed the coil - at the tfi with a test light, and as the car misses and timing light flickers and so does the test light at the tfi. only the coil- does this, the ign ground, and ign pos, (at the tfi), remain constant throught the misfire. i have verified the pip signal at the tfi and at the breakout box with my snap-on lab-scope. also have verified the spout signal at the tfi. any way replaced the tfi twice and the pickup twice, still the same. one last thing with a noid light on an injector, i can't tell if it flickers during the miss as the light is lowlit and not a brite led. i say this (about the injectors, because when the car misses very hard the 02's and th wide band jump lean for a split second.

anyone who is an expert on the eec-iv system or knows one i could really use the help.

[TwinSCs]

I noticed you replaced the FP relay, how about the EEC relay?

[Paulie]

Have you tried running without the MSD box hooked up?

[pedal92]

yes i have tried it with 4 different brand new msd boxes, i've tried it without any msd box hooked up, i've have three different ecu's a9l, x3z, and a3m1. two quarter horses, one is brand new. 3 brand new coils, i've used two different brand new modules, two different brand new pick-ups. two different tfi module harnesses. i'v checked all the wires including the 22k ohm resistor and all check good. i have even added 6 new grounds. here's the odd thing-- on a manual trans car, with the spout connector removed, the tfi module is supposed to be the only thing that fires the coil based on input from the stator(pick-up). it misses with the spout in or out)
I've probed the tfi harness on the ign pos wire and the test light did not flicker during the miss, i probed the ignition ground at the tfi and the test light did not flicker during the miss. i probed the pip signal at the tfi with my snap on, and from the graph it looks normal even during the miss.
if it was solely an ignition miss wouldn't the afr and o2's show it rich instead of lean during the miss or am i wrong on this.
one other thing when i switch from a signal test to a frequency test at the tfi on the pip wire the frequency goes really low and flat during the miss. however the signal and wave form are good.

[Paulie]

if it was solely an ignition miss wouldn't the afr and o2's show it rich instead of lean during the miss or am i wrong on this.

Oxygen sensors (even widebands) measure oxygen. When there is a miss, even an ignition miss, the oxygen in the cylinder is not used and goes out the exhaust and the excess oxygen make the oxygen sensor read lean.
Does the problem only happen at idle? Does the car run good the rest of the time? can you post a datalog and a copy of your tune?

[decipha]

sounds like a cracked plug, might as well go ahead and change them

remember, anytime you drop a spark plug while your trying to put it in, 9/10 it will develop a hairline fracture in the ceramic which doesn't become known until it gets heat in it

[pedal92]

decipha, i have already changed plugs, three different sets.
paulie, it started randomly at light to moderate cruise, every once in a while it would be at wot, some times on aceleration, now it has worked its way to idle also. i have a few datalogs that i'll post when i get home. the only thing i saw in them that caught my eye was this, the rpm drop that the tachs were showing was also seen by the computer. which is odd let me explain the big autometer shows a 200-400 rpm drop during the miss, the factory tach shows a 300-600 rpm drop (and this tach also bounces) the computer sees a 200-400 rpm drop during the miss. here's whats odd, the car is manual trans with a ceramic clutch (so even if the engine stalled it would turn constant rpms to match the speed of the car). at 55 mph in 6th gear 400 rpms is about 5 mph, and when the tach's are dropping out, the vehicle speed does not fluctuate (what i'm saying it's like the car is just losing the tach signal for a split second and the actual rpm of the engine is not dropping what the tachs are showing). anyway paulie thanks for clearing that up about the afr's i've been asking that question for 2 months now and no one even had a clue.

[decipha]

at 55 mph in 6th gear 400 rpms is about 5 mph

edit, FTL

sounds like its dropping power

step 1> replace ecu and fp relay
2> reground ecu, battery, chassis, block


step 3> enjoy

could be a failing pump

i still stand by what i said 4 months ago

[pedal92]

ok
new fuel pump areomotive 340 plus ALL new fuel pump wiring and relay . no change same exact symptoms..new in car $230 autometer fuel pressure gauge - NOT a fuel issue.

3 known good ECU's And 2 brand new quarter horses - Same exact symptoms

3 Brand new coils - same exact symptoms

3 new TFI modules - same exact symptoms

3 new hall effect pick ups - same exact symptoms

grounded six ways from sunday with new grounds - same exact symptoms

problem is the coil circuit is missing a ground cycle for some reason

heres a couple data logs and the tune

[pedal92]

here's the tune and a couple more logs

[Paulie]

I'll look at your datalogs later. Have you replaced the EEC power relay? Also, have you checked the fusible links by the starter relay? How about the distributor cap and rotor and plug wires?

[pedal92]

I have changed the cap, wires and plugs twice. Same exact symptoms.
I will change the EEC relay today or tomorrow. The EEC relay also is in the line to switch power to the tfi isn't it. Paulie, one thing I found was that a lab scope of the pip showed a nice square wave like it was supposed to be even through the miss, but a frequency test of the pip show major irregularities during the miss. Is this normal or is this a concern.

[Paulie]

If the waveform looks good, I suspect the change in frequency is due to the change in engine RPM. The ignition coil and TFI module are powered by the same circuit as the coil side of the EEC power relay. Fuse link N is located next to the brake booster.

[StangD]

I suspect the following has already been tried, but have you tried disconnecting all unnecessary devices such as tach, windows switches, etc? Anything that might drive off that tach wire circuit.

[85GT]

I have changed the cap, wires and plugs twice. Same exact symptoms.
I will change the EEC relay today or tomorrow. The EEC relay also is in the line to switch power to the tfi isn't it. Paulie, one thing I found was that a lab scope of the pip showed a nice square wave like it was supposed to be even through the miss, but a frequency test of the pip show major irregularities during the miss. Is this normal or is this a concern.

Try repeating that with the timing fixed. You maybe seeing the rapid spark advance variation as it tries to maintain idle speed.

[pedal92]

here are some videos of what the car is actually doing.

[pedal92]

last video

[cgrey8]

Based on the videos, it looks like the signal to the coil is literally getting affected/interrupted thus this could either be a pickup issue or a TFI module issue.

There's no way the ACTUAL RPMs are changing that fast. If the engine misses were associated with the coil, plug wires, plugs, or fuel delivery, the tach wouldn't drop so fast and dramatically. The tach would simply drop at the rate of actual engine RPM drop.

Since the tach is triggered off the TFI module, you can assume that what's failing to get the signal to the tach is also failing to get the signal to the coil. So my bet is the problem is somehow related to the distributor. I'd start with either the pickup sensor or the TFI module. The only other thing I can think it might be would be an intermittent power connection to the TFI module which could be anything from a bad connector, bad wire, bad ignition switch in the steering column, or any other component between the battery and the TFI module.

[pedal92]

thx cgrey, i'm getting ready to check some of those things. i'll let you know how it goes.

[pedal92]

cgrey i haven"t had much time to work on the car, and when i fix it i'll let you know. however i have one question, when i unplug the tfi and check the lowest wire (ignition ground back to the ecm, from battery pos+ it only show 7.5 volts, if i plug the tfi back in and backprobe that same wire i get full 12 volts. do you know if this should be 12 volt full ground with the tfi module unplugged or not. this is with the key on. if so does the pcm have a specific ground to the body for the ignition only or coul all three of my ecm's be faulty inside?

[cgrey8]

The 7.5v without the TFI connected and then 12v with it connected is odd. Where is the voltage being made up if what you are probing is the supply voltage to the TFI? And as for a dedicated ground for ignition, I don't know. The only dedicated ground I recall is the HEGO ground. But theoretically if you have solid grounds, a ground is a ground.

[pedal92]

cgrey its not the tfi voltage i'm checking but rather the ingnition ground back to the ecm. hooking the positive lead from the voltmeter to the battery positive and the voltmeter negative to the unplugged tfi harness (bottom wire-ign ground to the computer) with the key on so the computer is grounding the ignition wire = 7.5 or so volts then simply plugging the tfi connector in and just backprobing the same wire with the volt meter negative - 12.5 or so volts

[markedforlife]

Check your ignition switch on the steering column, disconnect it pull it off and see if
the aluminum case is separating from the plastic, there was a recall on these back in the day.

[pedal92]

i seem to have it fixed. after trying and changing everything, i broke down and bought another msd distributor, installed it last night and it seems to be fixed.

this is supper puzzling to me because i changed the hall effect in the other dist 2 times and the tfi module 2 times, which did not fix it.

the only thing i found interesting was, when watching the hall effect wave form an the lab scope i would have random voltage spikes on the rising edge of some of the wave forms for the hall effect.

the injectors never miss fired but the negetive side of the coil did. the shaft and bearings were fine in the old distributor, no play or wear. the old distributor only had about 600 miles on it when the miss started.

my conclusion is since the injectors are firing on the falling edge the voltage spike it never effected them, however the tfi is firing on the rising edge, thus only the coil misfire.

my opinion on the voltage spike....... the alternator (which was changed twice) cannot spike voltage as high as what was showing on the lab scope (35-45 volts) and was fine. also a bad ground cannot cause a spike that high. the only guess i have at it, (and could cause a spike that high) is that the distributor may have been magnetizing for a split second. this is just a guess.

because this problem was so indepth and i don't like the answers of just change this or that and see if that does it, maybe some of you may have a better idea of the cause or a way to diagnose this in the future...... For those who may have this issue in the future it would be nice to find something in our forums on how to diagnose it other than change this or that.

thank all of you who posted and commented trying to help me with this problem.

lastly the problem seems to be fixed, if it reoccurs i'll let you all know.

[pedal92]

Still Good.