hassgrant

I have a 91 TPI 305 Z28. My problem occurs during closed loop. When I press on the throttle, it feels as if my car takes a gulp of air and then tries to accelerate, and does so in a bogged down fashion. If I slowly accelerate, the vehicle feels somewhat normal. During open loop, throttle response is perfect. I have changed the O2 sensor, the EGR valve, the EGR solenoid came from a 91 305 Caprice, replaced all vacuum lines and gaskets, replaced TPS sensor, replaced IAC sensor, replaced distributor cap and rotor, tested MAP sensor and is in working order, fuel injectors test out ok and are custom aftermarket, and I am not throwing any codes except for EGR system occasionally (usually during normal driving around 70 MPH). The car also idles fine and starts up fine. My MPG has also dropped off significantly, but I think that is because it takes more fuel to accelerate. So any ideas on what the issue is? Could it be a clogged CAT or a timing issue or a fuel pressure issue? I haven't tested timing or fuel pressure at this point as I found my timing light destroyed and I don't have a fuel pressure gauge. I am just stumped that in open loop, besides a slightly high idle because it is cold, the car runs perfectly with great throttle response, but once it enters closed loop I have the acceleration issue. Please help, I have been chasing this gremliin for months now.

Jcase350

Sounds like the exact same problem I had several months back. Come to find out on the bottom side of the plenum there is a little bump that matches up exactly to where the vacuum line comes out of the top of the EGR valve that was cutting off flow. Just need to rotate the line 180 degrees to avoid that. That with replacing my injectors...finally got the problem to go away. I tried everything u described and finally had to take it to an auto shop for them to troubleshoot. Might be worth a look.

hassgrant

Thanks for the info. I will try that this week. I am also looking into the possibility that the auto parts store sold me the wrong EGR Valve. They have two that match my vehicle (OEM 17090106 and 17090105). I am wondering if one is positive pressure and the other negative pressure. I need to research the forum for the proper valve for my vehicle. I will let you know how it turns out.

hassgrant

Well I have not been able to find, with certainty, if my 91 TPI 305 manual transmission Z28 should have a positive or negative EGR Valve. Currently I have a negative EGR vavle installed, EGR114D from Advanced Auto Parts. I am thinking of just parting it out for the positive backpressure EGR Valve EGR115D. Also, I had to use an EGR Solenoid from a 91 305 Caprice, VS18 instead of VS19 (I believe). If anyone has any further information to help me with this situation, I would greatly appreciate it.

hassgrant

Well after thinking it was the EGR system, and then the O2 sensor, I think I have finally found the problem. I have some injectors that are either really clogged or not firing at all. I have one, the second from the front on driver side, that feels like it is not clicking at all like the other seven. Car still runs great when car is cold, so I am thinking the injectors are working when cold and then fouling up when they heat up, which was making me think it was an open/closed loop issue, and thus, an EGR or O2 sensor issue. Ordered remaned Bosch IIIs and will update when I replace them. Also ordered MSD wires, coil, distributor cap and rotor, and new ACDelco plugs.

hassgrant

Well, not the injectors, the timing. Checked my timing and it was set at 2 degrees BTC. I set it to 10 BTC and it runs great. Now to change plugs, wires, cap and rotor, and coil. Put more life into the car.

hassgrant

Well what I thought was 10 BTDC was actually 16 BTDC (end of gauge). After changing plugs, wires, cap and rotor, car still has acceleration issues and now seems to be hunting for idle. Occassionally she will fall off and then die. I changed the IAC as it was still under warranty - no help. I need to look again for vacuum leak. I am concerned about such advanced timing but she ran better at 16 than at 6, 8, 10, 12, or 14 BTDC. Any suggestions?

Rolling Thunder

Well for starters on the EGR valve either type of EGR valve positive or negative can work as long as it correct for your application. On the timing, depending on your AF ratio (which sounds to be out of whack) the time it takes to effectively burn the fuel air mixture can vary. So under these circumstances it may very well like the timing being advanced like that. However it also depends how you measured your timing? Did you measure it correctly with the EST line disconnected? Because if not the computer controls the timing if your trying to adjust the timing while the ECM is also adjusting the timing who know whats really going on then. On the flip side if you did do it correctly and the base timing likes to be at 16 btdc then maybe your ecm is retarding your timing for some reason which is causing this mess and also why it likes to be advanced so far. Still, even if it likes advanced timing, set it stock because if things arnt how they are supposed to be in the first place it could make troubleshooting your problem much more difficult than it has to be. On a side note you mention no mods to the car im assuming everythings stock?

My first suggestion is to hook up a vacuum gauge and see what your manifold vacuum reads (open and closed loop). Note the reading as well as the behavior of the needle (things like is it steady does it rapidly vibrate +/-3 hgs from the average ect). The vacuum gauge, while almost completely forgotten about today, is a great tool because it will give us a nice general overview of how your motors running. Hopefully this test will point us in the right direction. Again though you need to ensure the timing is correct to factory spec because your vacuum reading is also sensitive to your ignition timing.

hassgrant

Thanks for the reply Rolling Thunder. I will try and pick up a vacuum gauge in the morning because the dreaded uterance of 'vacuum leak' continues to come up as a possible culprit. I will also reset the timing to 6 BTDC - and yes, with EST unplugged using a timing light connected to number 1 plug. As far as mods, the car is stock sans a K and N oval air filter attached directly to the TB, a FlowMaster muffler, a 52mm throttle body, MSD coil, MSD rotor and cap, and MSD spark plug wires. Everything else is stock that I can think of. I will post the results of the vacuum test once I check it. Again, thanks for the reply and hopefully I can finally coral this gremlin that keeps sneaking in and giving me heartache.

jeremymsl

Try adjusting your tps sensor, it needs to b at .54 volts, mine did the exact same thing (idk about if in open or closed loop, im new to this so i dont even know what that means) but mine was at like .34 from a bad tune and that fixed it.

ninetyone

iTrader: (2)

Egr doesn't matter too much at WOT. 1991 cars don't have an adjustable TPS.

ASE doc

if it runs well in open loop, its not a vacuum leak, timing or injectors. The one part of the system that really changes in closed loop is the O2 sensor. Virtually any other problem would also affect open loop. bercause you've replaced the O2 sensor is no assurance that it's okay. To check the sensor, you either need to monitor data as the system goes closed to see if O2 voltage goes low or high or is lazy. Or tap into the O2 circuit with a voltmeter to watch O2 voltage. Or you can unplug the sensor and see if the problem goes away without O2 input.

Ive seen this problem a few times and have found a a few O2 sensors to be bad out of the box.

DJK90IrocZ

Hey Hassgrant,

How did you find out about those 2 EGR valves to determine which one was positive and negative pressure?

If I understand you...EGR114D is Negative and EGR115D is positive...correct???

None of the numbers on the valves that I have seen have the "N" or "P" in them.

I currently have the issue of in closed loop, the code 32 shows up with both of these EGR valves that I have installed.

Anyone have any ideas???