archangelus2

Hey Guys,

Well after puting up with my car acting up for about 3 weeks I am now fed up/scared and need some advice.

The problem: For the last 2 weeks my car has exhibited the low to mid rpm (1500-2500) stubmle/hesitation that I believe is talked about so often on these forums. It seems to come and go every time I drive the car with no dependencies. This hasn't been a "big" problem...the car has remained drivable and I just figured I would get around to checking the opti/ICM/coil/plugs/wires when I had the time. But what brings me to post this before I did all that is that for only the second time in these two weeks the car has had the rpm jump or surge upwards another 200-500 rpm (then immediately fall back to where it was), and this IS exhibited in the tachometer and on my scanmaster rpm display. It is very unnerving and makes me feel like my car is coming apart on the inside or something. It did this only today on the way home from work and once a couple weekends ago, other than that it has just been the hesitation/stumble but the rpm's have always appeared as I have come to expect them.

History of work recently done on the car:

1. MAC headers and Replaced Heads beginning of summer.
2. 3 weeks ago I changed the radiator fluid, put in a new thermostat and fuel filter, and hooked a scanmaster up for the first time. (After this it ran better than it ever has for about a week). Also seemed to get rid of a very subtle backfire I had developed.
3. 2 weeks ago I let my gf drive it (6 speed) in a parking lot and she stalled it 2x but other than that was fairly easy on it. (this is when the problems started)

If anybody has any specifics that I should check first let me know.
I am fairly familiar with most of the suggestions on this website but haven't seen anything on a dramatic change in rpm's like I am experiencing.
Also, throughout all of my work I have followed this site and the GM manual to a T.

Thanks for any feedback,

-Gabriel Terrel

95BlackTA

My car did something very similar and it went on for over 2 years...ended up trading it in on a brand new Nissan and never looked back.

Replaced opti, coil, plugs/wires, 02's, IAC, fuel filter, cleaned air filter/tb...never fixed it.

Good luck. Hope you can find yours out. Someone on here mentioned one time that theres somekinda head sensor on the head or something that can control where the RPMS are at...maybe someone else can explain that more.


EDIT: Check o2's..they can make the car run like crap.

archangelus2

One of the O2's is new (bosch).

The readings on my scanmaster seem to be ok for them. Of course they are all over the place but if I do a log at >75% throttle they usually come in in the 800's.

No other ideas or feedback?

Injuneer

There's no "head sensor" to control RPM. The PCM will increase the idle RPM for a low coolant temp, but you would notice that because the PCM would be going out of closed loop at temperature readings low enough to cause increased idle.

The PCM will also keep idle RPM elevated if it thinks the throttle is not fully closed (TPS >1.00V) or if the VSS signal is not "0", indicating the vehicle is still moving.

Have you checked the engine and PCM grounds? Have you checked the two connectors in the Opti harness for corrosion or damage? Have you checked the water pump weep hole for seepage, also the t'stat housing for leaks?

archangelus2

Briefly, where exactly are the engine/pcm grounds located?

ShowStopper540

I know we all have our different oppinions and whatnot but damn bro....you traded in american muscle for foreign trouble? you thought the problems in your domestic were bad ....you ignored one VERY important component, the ICM (ignition control module)...you're very right about the O2's, but if they check out fine id go with the ICM or the Opti.
I just finished chasing this ghost and for safety reasons i replaced the both of them rather than one or the other, so far so good going on 3 weeks.
These cars have many components to an ignition system, and replacing alot of random parts is just going to cost you money and hastle, judging from your symptoms (i had similar, jumping of the tach, sputtering, cutting off randomly) id go with what i mentioned earlier, the ICM or Opti, good luck, if you need anymore info, get in touch with me.

kainZ

You should have done a "Search." I had the RPMs jumping symptom not long ago. That was even in the title of my post. It was the 2nd time I've had to change the optispark. The wiring harness going to the opti was saturated with oil, which had also leaked past the aged wetpack seal on the opti harness connector, and into the opti, causing the opti itself to also begin to fail.

Anyway, to make a long story short, there are only a few places that can make the tach jump around. Opti, ICM, ECU, and the wiring between them. I'll put my money on the optispark, because that is the most prone to failure on our cars. Btw, if you get MALF 18/36, it'll be confirmed, and your car will start to exhibit stalling conditions, due to the ECU cutting fuel to keep things from blowing up.

Anyway, confirm the opti, then fix it. And don't post without searching with the search function first next time.

Good luck.

Ps. I lied, the tach can also jump as a result of the tach wire going to the dash cluster being bad, but that would be a benign failure, and wouldn't cause your other symptoms.