So I recently puchased a 93 Z28 with just over 169k on the clock. Ive owned it for two weeks now and it has a few issues.

1st. The car has a random low idle. Sometimes I start it up and its perfectly fine and other times it sits just under 500rpms. The idle smooths out after I drive for a few minutes ( sometimes less ) but it still bothers me somewhat.

2nd. The day I got the car, when driving on the highway I suddenly had a complete loss of power and the car backfired quite loudly a few times. I got off the highway and took the backroads and it did this 2 more times before I got to work.

I figured this had to be an issue with the MAP ( Since my previous car, a 88 Trans Am GTA had similar issues and it was related to the MAF ) so I popped the hood and checked the MAP sensor visually. The pigtail was missing its clip that holds it in so I squeezed the pigtail in tighter thinking that it simply came loose. It was fine for a few days after that. Took it on the highway a couple more times to test it and I had no backfiring or loss of power.

Thinking it just needs a new pigtail, I head out on the highway again after I get off work. After driving 60-70 mph for probably 15 minutes, I lose power and horrid backfires again. I pulled off the highway and squeezed the pigtail in tighter just like the 1st time, hopped back on and the car ran fine the rest of the way ( About another 5 mins on the highway )

Then just the other day, I was coming home and it lost power and backfired ONCE under hard acceleration ( Near WOT )
I've been taking the backroads to work ( only takes an extra 5 minutes or so ) and it drives fine when I'm not on the highway. Hell I even won 2 races ( 93-01 Integra and a 94-95 Mustang GT ) without incident. But not being able to take the highway is REALLY bothering me. Its fine that I have to take the backroads to work, but taking them to other nearby cities takes so much longer and so much more gas.

The car never died or shook. Just acted like I let off the gas and when power returned, it was accompanied by loud backfires. ( I even think it shattered the inside of the cat since it sounds like it has rocks in it )

I'm thinking the pigtail needs to be replaced along with possibly the Ign Coil and both of the 02 sensors after doing some reading. I'm REALLY hoping its not the Opti. I haven't checked the wires yet but I'll be doing that before I purchase anything.

 

Firstly, welcome to the board....

I have a 94 and there are significant differences between 93 and 94 so please bear that in mind....

A clogged cat can cause low power and slow idle. It would not be intermittant, though.

When I hop on the way back machine, I remember having problems with my MAP sensor. It would always set a code and run normally after that. the speed density cars, like your 93, could be more sensitive to that. It never caused a power loss and a backfire.

Back on the way back, I had a car that had a bad coil wire. The connection would break and the car would die momentarily. Then it would catch again, usually before the engine Stopped turning. The restart always created a big backfire and ruined a muffler.

The opti has a bunch of ways it can fail but I haven't heard of a combination like yours.

So, after all that noise, here's what I suggest:

Do a scan of the computer to see what's going on.
Replace the pigtail on the MAP sensor.
Check the coil connections, including the grounds on the head and chassis.
Check the fuel pressure.
Check the plug wires. If you don't how old they are, just replace them, along with the plugs.
Check the EGR valve.

It's possible that it's a combination of things. For example, the egr could be causing the low idle speed while a coil wire could cause the shut down/ restart issues.

After you gather that info, repost. There's lots of smart folks on here that can help you. For a start, there are two sites you should bookmark. One is shbox.com. Rob has an encyclopediia-like quantity of information with a lot of pictures to help with locating items. It's worth going there just for the parts list. The other site is injuneer.com: Fred's Formula V8 Page. Injuneer has a great understanding of the ECM/PCM and explains it very well. Both of these sites are by guys that actually turn wrenches on their cars so it isn't merely academic. Besides those gents, there are quite a few guys on here that can help with all aspects of performance builds and troubleshooting....

Good luck!

 

Forgot to mention in the OP that the car isn't throwing any codes. Thanks for the links, I'm sure they will come in handy.

 

Did you do a scan or just say that because there is no service engine soon light illuminated? I ask that because there are faults that won't set a light. It's just a gift that GM gives us....chuckle. Luckily on a '93 car you can flash the codes using a jumper wire on the ALDL connector. on 94+ cars you can't...

If you do a search you should be able to find out how to do that. If you have any stored codes you'll get them that way and knowing that there are no codes at all will help the diagnosis.

If you need a place to start, really inspect the wiring to the coil and ICM. They're located on the front of the drivers side head. You may have to remove the intake ducting to get a clear view...I forget as I switched to Ram Air a long time ago. If everything looks good, start the engine and flex the harness a bit to see if you force the fault.

 

BW has covered some of what I'm about to mention.

That really sounds like an Opti or ignition issues to me, but best to let some others more experienced that me chime in...a plug inspection is certainly something to consider.

The 93 is purely speed density, so there might be some small something we are not thinking about.

My brother had a cat fall apart on his '99 3.8 V6 Camaro. It was really down on power and would not rev much above 4000 under high load. No backfires, no SES codes, and it idled just fine when it happened. We could smack the bottom of the cat and hear parts rattling around. We changed the cat and it was like we had added 100hp to it(truth be told I bet it did....).

 

The loose MAP pigtail is probably causing major issues. Even with the side clip intact, the harness connector tends to dry out and crumble, and cause problems with intermittent MAP readings. With an MAF car, it's not as big a problem, but with your speed-density 93 it's going to play havoc with the engine operation.

 

2 things people tend for forget. The coil is a tune up item and should be replaced periodically. And so is the cap and rotor. I'll bet if you look inside the cap, you'll wonder how the thing ran in the first place.

 

Sorry it took so long to get back but yea I know how to check for codes on these older F-Bodies lol.

I've checked multiple times and theres no codes. My GF surprised me and bought me a new ignition coil anyway so I said what the hell and threw it on. Car was fine for a few days again but it did eventually do its schpiel of ignoring the gas pedal and backfiring on the highway, though this time, I was less abrupt and only backfired once so I'm inclined to believe the Ignition Coil helped the issue a little.

Unfortunately the ignition cylinder took a complete dump out of the blue ( drove home from work, came back outside about 20 mins later and the cylinder was completely locked ) so the car is currently sitting at the local Chevy dealer with a $450 bill attached to it.

The service manager said he would take a look for free and see if he could figure out what my issue was that I described in this thread. I'll post back with his findings when I find out.

p.s. and yes I tried jiggling the key, turning the wheel, the whole 9 yards. Service manager said the cylinder was pretty worn out and needed to be completely replaced. As for the Cat, I'm going to have that taken off for the time being since I don't have to bother with emissions until the end of May. The car doesn't feel like its down on power ( Its a hell of lot faster than my 88 GTA was and it was right on par for a good running stock L98 ) but we'll see the difference when the cat gets taken off.

 

Sorry I have taken so long to reply here. My computer was out of commission for a while.

Had a diagnostic done on the car and turns out it just needs a tune up. Wires/Plugs + Cap/Rotor. All the sensors were checking out ok and the engine was running good aside from the aforementioned parts needing replacement. Got quoted at over $1400 out the door. I laughed and told them hell no, I'll do it myself ( which was my plan anyway )and the mechanic laughed with me and agreed that at such a steep price, its much better to get it done privately or DIY.

Just wanted to post this incase someone else has the same problems later down the line and is looking for answers instead of blindly throwing parts at it.

 

While I hope this solves the problems, I'm not really buying it....no offense. I haven't seen too many instances where a car is running fine, then loses power, backfires, then runs OK again and it ends up being a tune up. When those replaceable parts degrade the car just runs poorer and poorer. I believe there has to be a direct cause of the power loss that can be affected by movement or vibration or something....

 

I agree with bw hunter but I also think you have multiple issues based upon experience ive had with my 96 and my 93 truck. If you still have stock wires they are old and cooked you will notice a seat of the pants difference as with new plugs. I changed mine out at 140,000 and at same time had injectors"cleaned" huge difference in how car ran. On my truck ive had a wire issue that was unnoticeable until truck got to temp and I hadn't been getting up to temp much and by the time I found it I had warped a valve. Ive also had heat sink type issues with the pickup coil so perhaps your coil was one aspect of multiple issues. Ive also had the cat plug on my truck(or muffler) and it was like running with a jake brake. I couldn't climb above 45 mph and the more gas I gave the worse it ran(while limping home) ive also had ignition control modules start losing power before that died(might be the only guy who this ever happens too) but I carry a couple spares in my truck and have swapped them and noticed differences on cold morning startups and performance as well as had three go out since 93. The dealers diagnosis may not represent real world issues as perhaps different things respnd different under load or after it gets hot(not motor reaching temp). Another thing I have heard is heavy key chains with the swinging and weight will wear out ignition cyllnders faster and grabbing the wheel to pull yourself in effects things in the colum especially tilt wheels. Be optimistic about replacing those items and don t forget locktite on rotor screws but know it may not solve everything.

 

my first 93 z-28 that i had had similar problems found out the coil pack had a small crack in it. just saying.