Jessicaj91

New here and hopefully can get some help. I have a 1991 z28 with the tpi 350 and 700r4 in it, has 114,000 stock miles.

In 2012 I was living in a different state and went to take it out for a drive, as soon as I pulled onto the street, there was a loud pop (like a misfire but hasn't done it since) and then it drove fine until I came to a light and had to stop, then it stalled and died as soon as I slowed down. So I started it again and it started up fine but died shifting into gear unless I gave it gas while shifting. Now it starts up decently but once it starts warming up, the idle starts to bounce around in park and neutral between 1000 and 1400, but will go down to 500 and try to cut out then surge up to 1500. Putting it into gear will kill it 90% of the time unless I give it gas while shifting into gear. Once in gear, it may or may not stay started. If it stays started, the rpms surge a little up to 1500 then bounce around again like in park and neutral and will usually die. If you drive it, it will stall out every 10-40 feet and die then continue to do that until you park it and let it sit for a while.

I've tried pulling codes but it only flashes 12. I've changed out the IAC, throttle body gasket, thermostat, oil change, and TPS recently. Traced vaccum lines and can't find any leaking ones. Cap, rotator plugs and wires were done a couple years ago and I plan on doing plugs again once I can get it up to get the passenger side ones out, I haven't driven it a whole lot since the first day I had issues in 2012 (I've only put about 20,000 on it since 2007 anyway), but always start it every few weeks. It's not overheating or anything, oil pressure stays in a normal range but I've noticed it will drop once the engine warms up and it's getting ready to stall out. My neighbor is a senior master mechanic and I've had him come look at it a few days ago, he thinks the torque converter might be locking up, but he's not 100% sure since it won't stay started long enough to take it for a test drive more than about 30-40 feet tops, and he's stumped as to why it's acting the way it is and not throwing codes. Any ideas on what to change or look for next? Thanks for any info and help!

mcgarnicle

iTrader: (14)

Sounds like an excessive vacuum leak. The SD cars aren't as sensitive to vacuum leaks as the MAF cars, but usually a bouncing idle indicates that the computer is swing adjusting because unaccounted air is entering the system. Looking at vacuum lines is only the 1st step, there are a lot of places to have leaks on a TPI. The best way, hands-down, to find a vacuum leak is with a smoke machine, it pumps smoke into the intake and anywhere smoke comes out is a leak.

Jessicaj91

Thanks @mcgarnicle, I'll try that.

vinny R

It sounds like you may have more than one issue going on here. Does it smell rich? rotten egg smell? The dying going into gear could be a vacuum leak but the smell usually comes with said leak.
I had the same surging up and down in the same RPM swing as you. That turned out to be my TPS was sending the wrong voltage to the ECM. You should check this with a voltmeter, don't take for granted that just because you replaced it is working properly, I am speaking from experience here. You can search this site for the Test and the voltage.
The backfire, stalling out and restarting right up and stalling shortly after sounds like your IAC. The control module in the bottom of the dizzy. I just got done fixing this issue with mine and man did it take awhile to trace it down. It controls the timing and when it gets hot it starts to malfunction and most of the time once the engine cools down some it goes back to working. I have gone through 2 of these in the past couple years and that is one reason I did not look at this part because I already replaced it! Hope this helps.

Jessicaj91

Thanks for replying. It's only smelled like rotten eggs once for a split sec about a month and a half ago and I've never gotten it to do it again. The IAC and TPS were both changed out about a month ago. I replaced the connector as well with the iac since the original one was cracked. I've checked voltage and it's fine. I've changed the tps out before because it threw a code for it and it was surging bad but only when i was stopped and it would go up to almost 2000 then back to normal. It doesn't run like it's missing, just the crappy idle. I'm thinking about checking out the ecm this weekend and I'll check the control module as well and also see if i can't fog it for leaks

Jessicaj91

Oh and it doesn't smell rich at all, and no smoke

vinny R

Sorry I meant ICM...Ignition Control Module. It is the electronic part that is in the bottom of the Dizzy. I think you can take it out and have a parts store check it but as I said it usually fails when hot so it may read o.k. when tested cold. This has to do with timing so I would check the base timing also.

Jessicaj91

I'll check it out. If it's bad, do you recommend any particular brand?

vinny R

I always try to buy AC/Delco when available and this part you can get an AC/Delco.

Go Blue

Do you have a MAF or a MAP sensor on your car? It really does sound like a leak after the MAF sensor or like the MAP sensor might have blown as either 1 is getting a bad reading. If you have an air leak after the MAF sensor, there is air getting in that isn't being accounted for and the oxygen sensor is trying to lean out the car and killing the engine in the process because the MAF isn't accounting for that extra air. You can take carb cleaner or brake cleaner, start the engine and let it idle, and start spraying a little at a time starting before the MAF sensor and the work your way back to the engine and around the throttle body and intake. When the engine surges or dies, you have found your vacuum leak. Replace that gasket.

mcgarnicle

iTrader: (14)

91 would be a Speed density car. MAF cars are more sensitive to vacuum leaks than SD cars, but if it's a large enough one it would cause the kind of issues she's experiencing. If the car will idle or stay running the brake cleaner method Go Blue described works well, but like I wrote earlier a smoke test is much more precise and my preferred method.

Jessicaj91

Ok thank you figured since you've replaced a few you'd have a brand you like better than others

Jessicaj91

I did the carb cleaner test before replacing the IAC last month, but it didn't surge or die at all. So I started replacing sensors that are known to go bad on them and testing others, but nothing yet. I'll try fogging it and check the ICM this weekend see if that comes up with anything

Jessicaj91

I have a video of the way it idles right after starting it up if anyone is interested

Go Blue

Jessicaj91

I'm kind of thinking it might be that, since above he stated that when they get hot they burn out, at the time I was living in Phoenix AZ when it originally started having issues. But I'll still fog it too just in case. I've had the car since 07, I'm the 3rd owner, but I ran into the 2nd owner at the store shortly after buying it from a dealership where it sat in a back lot for about 3 years before I convinced the dealership to sell it to me. The 2nd owner said the car had issues after he let his kids tear into it, but didn't really go into detail. And honestly I haven't really had many issues with it other than replacing the battery and alternator when I got it since they were bad from sitting for so long, the tps went out once in 2010, did cap rotor wires and plugs a few years ago, and the starter went out once. But other than that it's been a good car

Go Blue

Jessicaj91

I cleaned the throttle body and replaced the throttle body gasket before I did the IAC. I'll try unplugging the battery tonight