I know this has been beat to death.... and I don't have a way to scan... but here goes. My stock 94z auto with 176k has the dreaded open loop stumble. It's been getting worse. Recently my water pump failed. When I replaced it I also put the opti from my 93 on. I had just replaced the cap and rotor on it, before I parked the 93, and it was working great on that car. I hoped that this would cure the hesitation problem on the 94. It did not. After all that work I am now determined to try to get to the cause of the stumble. I really don't have easy access to a scanner or laptop and my funds are rather limited. The stumble seems to be much the same as others have described. The car runs great in closed loop but once warm the idle gets a little rougher and until about 2k it's low on power. The warmer it gets the worse it gets. Closed or open loop once past 2k it'll pull hard to the red line. I tightned some loose intake bolts towards the rear of the intake and replaced the plugs with some autolite plats. The plugs I pulled weren't that old and showed IMHO normal wear, if any. I have looked under the hood at night and don't see any signs of coil or wire problems. I know this isn't a definitive test and when I replaced the plugs I almost did the wires too but $$$$$. The o2's looked old so I figured that was the most cost effective next step... I think I made a bad decision though and here's where I need some extra input. I bought Bosch o2's. (yea I know but that's all our local parts stores had). After I replaced them I took the car for a ride. There was no question. The hesitation was at least 95% gone, if not completely. Part throttle response was back and it just sounded lean n mean. My happiness was short lived. After 250-350 miles of cruising over the course of 2 days the stumble has pretty much returned. Sometimes if the temp isn't over say 190 it'll run a little better but nothing like those first 100 or so miles. Also before the sensor swap the ses light would come on, but only very intermittently, and it would clear itself up within, at the most, 2 or 3 minutes. At first I thought the ses activation was random, because with the old o2's it happened so infrequently. The Bosch's have caused to ses to get much worse. Now I can make the ses come on. Here's what causes it- I'll turn into my neighborhood which is loaded with speed bumps. I'm going maybe 30 mph. I'll brake for the first bump and slow to a crawl. Give just enough gas to get over the bump. The next one comes up in say 10 seconds. I'll do the same-a little gas and then over the bump. Once over the second bump it's clear for a good stretch. If I leave my foot off the gas the car coasts along as it should at maybe 1k and 5 mph. After 10 seconds or so I can hear and feel the car/rpms surge up and down just ever so slightly. You can't really even see it on the tach but it's there. Thats when the ses trips. I realize now that this happened with the old o2's but it was so infrequent I didn't catch the cause and effect pattern. As I said though, with the Bosch o2's this happens almost every time out. Damn GM and this OBDI hybrid crap. On my 93 the paper clip might be able to point me in the right direction. I've checked my TPS and IAT. In a nutshell what I'm wondering is this- did I find the real problem and the fact that I used Bosch o2's screw me? Or am I overlooking something- the two other things that come to mind are the plug wires or maybe just maybe EGR? When I bought the car 15k ago I did clean the EGR. The valve moves if I push on it by hand. I'm leaning towards replacing the bosch with delphi or denso. Any thoughts/input would be super appreciated. One last thing (wow sorry for the small novel of a post) I have changed the fuel filter but I'm not leaning towards fuel since the car runs great on the top end but I do have a gauge so maybe I'll check that. Thanks all.

 

Disconnect one of the O2's. See if the car runs better even when hot once it is disconnected. If so, dump the Bosch.

My GM vehicles will not run with Bosch.

Also disconnect the vac line to the EGR and test that as well.

 

So I switched my Bosch out for some ngk o2's. Did almost the same as Bosch. Tinkered around a bit with the EGR. Put new silicone on the ICM. Cleaned my grounds. Still running mostly like crap esp. once in open loop. When I removed the original delco o2's I noticed the one on the passenger side had markedly more carbon build-up on it compared to the driver's side. When I removed the Bosch's the same was true, even after only 2 days of service. I was under the hood again today (my neighbors must think I'm totally insane by now) but I will not give up, esp. since now I've sunk some of my non-existent cash into this endeavor. Started the car and checked fuel pressure which was ok. I blipped the throttle hard from under the hood and the car actually backfired a little through the exhaust. I listened really hard and it sounded like I had an exhaust leak somewhere near the pass. side rear of the engine. Thought maybe the EGR tube or gasket but everything looked good. I decided to re-purchase the plug wires I had returned to Autozone thinking maybe I had a misfire on somewhere on the passenger side causing the carbon on the o2 sensors and the exhaust leak from unburnt gas passing through a slightly leaky gasket or something. Well... once under the car for the 10 gazillonth time (of course AFTER pulling the serp. belt, roller wheel from the tensioner, and all passenger plug wires to get ready for the wire replacement and- hmmm all wires look good and ohm out ok-) I happened to be gazing up at the passenger side ex. mani. looking for blown gasket, the meaning of life, divine intervention, or whatever, and-there it is!!! Through the opening on the heat shield, where you can see the manifold iteslf (that little cast iron bastard) there's a big old crack between cylinders 5 and 7. I guess with my gutted cat and louder exhaust I never really caught it. Now the real PITA begins, I can already see that the very front-most bolt is missing. And yes virginia the piece of wire I stuck in the hole says it's been busted off. Is nothing ever easy? Wish I had money for some ghetto headers. Anyone want to donate to an LT1 guy who's down on his luck?

 

lol you pay the shipping and i have some eddlebrock shottys that need a beed of weld ran around by where the thing bolts to the block

 

appreciate the offer. where are you located? man I just got done removin' the busted manifold. the car gods and good vibes from all here were with me. no broken bolts exc. for 2 of 3 on the "collector" flange, which I kind of expected.
I can't wait to get it back together so I can feel the REAL power. I also have a 93z (speed density rocks) with a stock 93 vette motor that I had to park. Was runnin great before that. Let me know if anyone needs anything. Maybe I could trade for a few goodies.

 

I have stock manifold off a 95 Z sitting in the garage. If you want it...its yours. As long as you dont live in Siberia, I will ship it for you.

waters.j@comcast.net