Althought the car (1991 Camaro Z28 5.7 liter w/ A/T) functions very fine when it heats up even slightly after start-up regardless of the weather temp., (I have to drive only for half a mile), I have the following problem:

1) It starts fine and idles slowly but progressively to a higher RPM when idling and while warming up.

2) However, if I start driving the car , as soon as I release the gas pedal (say, I have to stop at a stop sign), the car cuts off. This happens only for half mile or so. Then all is good.

3) IF I do not stop, say within half a mile or so, this symptom ceases and the engine operates regularly as it should.

I did a major tune up so practically everything that needed replacement is replaced in terms of sensors and switches and plugs and cables and vacuum hoses, etc.

The only thing that is not OEM is the TB: I have purchased a BBK1539 (58 mm.) throttle body for my 1991 Camaro Z28 5.7 L. It is an upgrade from OEM 52 mm. TB.

I have also replaced the old TPS and IAC sensor and its water housing.

I also added an helix air foil since the bores on the intake is 52 mm.; I calculated that unless I forced some air through turbulence, 58 mm. TB would not make much difference. I tested the problem with and without the foil on and there was no change. So the cause of my problem is not the foil.


TPS adjustment is made at exactly 0.54 volts at idle (warm engine temp) and TB is adjusted to run at 750 rpm in Drive when warm.

5) IAC is a new part NIEHOFF 21773. Tested with the old one and nothing changed. I ruled that out as well.

6) I am also thinking about EGR solenoid. Although the connections are verified and proper, since the stock solenoid is no longer in production for L98 engines (1986-1992 Camaro 5.0s and 5.7s), I used an LT1 engine solenoid from a 94 corvette because it works on the same principle, e.g., it does not "bleed" excess vacuum (pre-86 engine solenoids) but simply turns it on-off. Can it be that this part is not compatible and not working according to OEM ECM specs?

Totally lost. Any ideas how to alleviate the problem?

Thank you in advance for your help.

 

My guess is the line going to the throttle body is just to pickup vacuum for the solenoid. When the ECM pops the solenoid, that vacuum pulls open the EGR with the second line. If that ain't right, somebody straighten me out. Negative pressure EGR had me screwed up in my thinking before. It may have again.

 

My car was acting like yours when the IAC was closed and disconnected. Best guess would be you IAC is stuck or otherwise not working.

However a 58mm TB is way too much for a stock motor, and could be causing you some greif. Stock TB is 48mm and will support a pretty warmed up 350.

 

thanks all for your replies. I managed to solve it, FINALLY! Here is the rundown:

Stock solenoid is no longer in production for L98 engines (1986-1992 Camaro 5.0s and 5.7s). I used an LT1 engine solenoid from a 94 corvette because it works on the same principle, e.g., it does not "bleed" excess vacuum (pre-86 engine solenoids) but simply turns it on-off. However, unlike OEM solenoid, both pipes have the same size so I did a trial and error by exchanging the hoses to attach it to the vacuum source (TB) and the second try gave proper result. So this is an FYI for you in case somebody might have the same question. All well that ends well

Thanks much for your time.

 

Actually I was in touch with BBK and they adviced using an air foil with it for 58mm to bear fruit with a stock engine and they said they haven't had any complaints for a 58 mm. replacement on stock TB for 5.7s. I guess I will see how that goes

So far the car works very well (especially after proper solenoid connection is established).

Thanks much for your time.

 

egr solenoid connection is fine but after extended period, (apparently the car was not cooled of enough to properly test it), the problem is back: It still dies off until I dirve it 1/2 mile or so. STILL LOOKING FOR IDEAS. Thanks.

 

hi i think the problem might be in your iac valve. there is a procedure i used when i had to replace mine. I cannot remember exactly how you do it but in the Haynes manual for your car it gives exact a step by step procedure to set the iac valve to work properly. Sorry i cant be more help

 

try putting the stock throttle body on it and see what happens

 

same, already did.

Anyhow, I isolated the problem:

I tested the solenoid by blowing into EGR valve connection when engine off. It flows unrestricted which should.

Then put the diagnostic thingy, turned the engine on without starting and causing the solenoid to close. Blew into EGR valve connection and it is closed. ( doing this test won't give proper results, I think, by blowing into TB connection pipe due to the existence of the bleeding valve behind the solenoid).

So the problem must be EGR valve itself. Because when I block the valve hose and/or the TB hose, the car works fine.

I will change the EGR valve and see.

 

Finally, the car is in tip-top shape so I am commenting in all my previous problems to help out people with similar and/or same problems: Yes, this problem was due to EGR valve and its replacement fixed the problem. It is difficult to change it though as one is required to take out the plenum.

 

im having the same problems with my idle ive gotten a full engine rebuild and and replace a lot of parts in it i have a 2.8 88 camaro 5 speed but on cold my engine idles low then dies if warmed up then engine is fine