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I have an '87 VG30, 180k miles, engine allegedly has 120k. Had problems with hesitation and hard starting. Replaced the dizzy, rotor, plugs, plug wires, and a couple wire connectors. Ran great for about 2 weeks, then suddenly have had worse hesitation as well as rough idle and a bizarre problem where if I stopped quickly at a red light or stop sign, it would idle for 1-3 seconds then the idle would oscillate. While oscillating, it would go from ~700 RPM (normal idle speed), then go up to maybe 900, down to 600, up to 1100, down to 500, and so on until the low point in the oscillation would cause it to stall. I'm not sure if the oscillation begins with an increase or decrease in RPMs. When it does stall like this it starts back up immediately with no problem. It doesn't always do this, and it seems to stop when the engine suddenly decides that it's warmed up. If I slow down to approach the stop sign/red light at a slower pace, meaning I take longer to go from full speed to a stop, it happens less often. When I'm just driving at above ~1500 RPMs she goes just fine--even better when cruising the highway at ~70 mph, though I do think the power during acceleration at low and mid speeds (does fine on the highway) is a bit lacking from what I would expect from a 160 HP car.

It usually starts hard, but sometimes (40% of the time?) it will decide to start up immediately, and then stall within a couple seconds. Usually takes 3-5 starts before she'll idle without stalling, and even then the hesitation is usually so bad that if I tap the pedal she'll stall. Idling for 1-5 minutes (depending on how cold it is outside) allows her hesitation to be not so bad that I can back out of the driveway and start sputtering down the road. Once she warms up the hesitation is mostly gone, and the hard starting is gone until she cools again.

I understand that the hesitation and hard starting problems that I had before the ignition tune up could be caused by a number of things (listed below), and will be replacing the MAF and TPS sensors, re-checking the vacuum system (replaced a hose that got melted a couple months ago, system was fine then), checking battery load and fuel pressure in order to make sure things are up to par. I also have what I'm guessing is horrible gas mileage, 14.5 mpg. This poor efficiency has been unchanged since I bought it a year ago, and has not improved with the above work as well as having new valve seals, head gasket, oil pump, water pump last summer. What is the normal fuel efficiency for this engine?

Also, I notice that the in-dash voltmeter dips and surges quite a bit during periods of heavy hesitation (dips to maybe 10 V, only dips when hesitation is so bad that RPMs dip to ~500) and the strange oscillating problem (10-14 V, I'd estimate). No doubt this is making things worse as I've read here that Z's don't like undervoltages. She has steady voltage when idling properly and driving normally. The battery is strong.

EDIT: The idle problem is much worse when it's raining or there's high humidity.

So then, to recap:
1) Cause of bizarre oscillations during idle?

2) How bad is 14.5 mpg for this car?

3) Does poor mileage and mediocre power at 700 to 2500-3000 RPM as well as the oscillating point to a more specific cause than the normal dozen or so possibilities for a rough idle/hesitation problem?

4) I would like to avoid replacing the MAF and TPS sensors since they're each ~$160 (via discountautoparts.com and courtesyparts.com, respectively) and I'm generally broke. Are there any other free/cheap things I can do first, such as resetting the idle A/F mix?

5) From posts in this and other forums, it looks like the possible causes for the rough idle/hesitation problem are:
mass airflow sensor, throttle position sensor, cylinder head temp sensor, crankshaft position sensor, EGR valve stuck/bad, vacuum leak, idle mixture needs reset, bad ground, clogged fuel filter, bad O2 sensor, leaky cold start valve, bad "thermotime"(?) switch, bad injector(s), leaky idle air bypass valve, or undervoltage problem (e.g. bad alternator or slipping alt belt). Did I miss any?
 

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Discussion Starter #2
I'm hoping that my post wasn't too intimidating. I'm basically trying to determine whether or not fixing this problem is worth it before I sell it off in a couple weeks. I bought the car last October with great zeal, but the overcomplicated troubleshooting required makes it seem that this might not be the right car for someone like me who is taking on their first project. Then again, maybe I'm just making things more complicated than it needs to be. It wouldn't be the first time.
 

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First things first you need to run the codes from your ECM and post them up here so we can see whats going on.

Locate ECCS control unit, Passenger kick panel right side.
Ensure mode selector on control unit is turned fully counterclockwise.
Turn ignition switch to On position and ensure inspection lamps on control unit light and remain lit. If not, check harness for battery power and grounds. If all power and ground circuits are OK, stop test, replace control unit and proceed.
Turn mode selector fully clockwise, and ensure Codes 23, 24 (Turbo only) and 31 are displayed. If codes 23, 24 (Turbo only) and 31 are not displayed, proceed to DIAGNOSIS USING CODES to verify components and circuits. If codes 23, 24 (Turbo only) and 31 are displayed along with additional codes, note the additional codes and proceed to step 5.

NOTE: It is possible to have false codes contained in the control unit's memory . If more than two additional malfunction codes exist in the control unit's memory, or any codes at all that do not have any relation to the symptoms the vehicle exhibits, it would be wise to clear the control unit memory and drive the vehicle for enough distance to recreate the code producing problem. At this point, proceed back to step 2 and continue.

Depress, then release accelerator pedal and ensure Codes 24 and 31 are displayed. If codes 24 (Turbo only) and 31 are not displayed, go to DIAGNOSIS USING CODES to verify components and circuits. If codes 24 (Turbo only) and 31 are displayed along with additional codes, note the additional codes and proceed to step 6.
For Turbo only, shift transmission from neutral through all ranges and back to neutral and ensure Code 31 is displayed. If code 31 is not displayed, go to DIAGNOSIS USING CODES to verify components and circuits. If code 31 is displayed along with additional codes, note the additional codes and proceed to step 7.
Start engine.
Ensure Codes 14 (Turbo only) and 31 are displayed. If codes 14 (Turbo only) and 31 are not displayed, go to DIAGNOSIS USING CODES to verify components and circuits. If codes 14 (Turbo only) and 31 are displayed along with additional codes, note the additional codes and proceed to step 9.
Drive vehicle at speed above 6 m.p.h. (Turbo only). Code 31 should be the only code left. Note additional code(s) and proceed to step 10.
Operate A/C momentarily and ensure Code 44 (System OK) is displayed after turning A/C off. Note any additional code(s) and proceed to step 11.
Turn mode selector fully counterclockwise and stop engine.
Perform diagnosis for codes displayed in steps 2 through 8 as described in ``DIAGNOSIS USING CODES.''
 

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My car use to do that a few years ago. just so happen that my fuel pump went out shortly after that i started seeing that. i replaced it and it fixed the problem not saying that it is the same thing that is happening to your car. but check the pump out. get a hold of a graphing multi-meter and an inductive amp clamp and check it out to see if the wave form that the pump puts out is consistent and there is no abnormal wave form. if you can do this and there looks to be anything out of the norm. take a picture of it and ill be glad to take a look at it to see if i see anything abnormal.
 

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Discussion Starter #6
Well, I started out by downloading and printing the 87 FSM (i had the 86 version before and didn't realize it). I read half the FSM, mainly the sections on ECEF, EL, and HA (i have some other problems related to the heating system). I'm really glad I did that, because I never knew how much diagnosis could be done via a car's ECU. I learned a lot about diagnosis! I now realize why my initial post was largely ignored--it doesn't make much sense now that I know how to diagnose everything :)

So I did what you said, and got only code 31 (it's a non-turbo). Then I ran through the ECCS diagnostics for all 5 modes. All diagnosis was done with the car warmed up to normal operating temp. Here are the results:

Mode 1: With the engine on, green would stay on for about 2-3 seconds, then off for 2-3 seconds. It repeated this cycle the whole time it was running, and red was off the whole time. My interpretation is that the mix is normal since there is no signal to indicate normal--there are only two types of signals, one means rich, one means lean, so for it to regularly switch between the two seems to me that it's right about where the mix should be. Kind of like how a multi-meter on DC voltage mode hangs around zero when you touch the probes together. Is this correct?

Mode 2:
-engine idling: green off, red off (this is an open loop condition. what does this mean?)
-engine off/ignition switch to on: green and red on (not normal for a stopped engine...)
-after engine stalls due to idle problem: green on, red off (normal for a stopped engine)

Mode 3: only code 13 showed up--the cylinder head temp sensor. I will go through the diagnostic to see if it's the sensor, the ground, or the harness today and report back.

Mode 4: (i was sure not to go from mode 3->4, which clears the ECCS memory)
no problems detected

Mode 5: no codes within 5 minutes. I was going between idling and short and long engine races (depressing the pedal to increase RPMs) during those 5 minutes. The car even started doing the oscillation thing during this without tripping any codes.

I will have to wait to do the fuel pump test. I have access to an oscilloscope and inductive probe at work. Hope to have the test done by the end of this coming week. Interestingly, the guy I bought the car off of, who is a professional mechanic, mentioned that he thought the fuel pump might be going bad a few months back when I had him replace the power steering rack (I'm not equipped to do that level of work). He said that a good month before the idle oscillation problem started, so I'm not sure how he would know this. Also, code 22 didn't show up in mode 3, so if there is something wrong with it, the computer can't detect it.

EDIT: Also I just realized that when I was doing the ignition tuneup I removed the FICD and idle up solenoid valves, cleaned them off a bit, and put them back on. I'll have to check to make sure I didn't injure their harness or otherwise screw up the valves in the process. I suspect that these are ok though, since the mode 4 diagnosis returned all ok.
 

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I'm going through this same thing in another thread.

Please don't drop the ball and disappear ;)

I just checked my fuel pressure and it seems ok.

Today, about 1/4 mile from home it goes into BURBBLING MODE, or Sputtering mode and wont move or rev.
I had to keep futzing with the throttle just to keep it from dying.
Eventually, it cleared up (after things fully warmed up I suppose).
 
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