95 Tahoe engine miss

[CatTech]

Hey guys, I have tried everything to fix my tahoe. Its a 95 with the 5.7 4x4. The clock shows 115k. I noticed a miss and that the engine "stumbbled" during take off. I figured it was time for a tune up. I did plugs, wires, fuel filter, cap and rotor. After this didnt fix the trouble I looked deeper. I found the ignition coil to be a bit weak so I replaced that....still didnt fix it.

I have checked the TPS, IAC, ignition module, fuel pressure, fuel pressure regulator. After I didnt find anything I gave in and took it to the local dealership. They said it was a bad ECM. After debeating the $700 I was quoted, I called Napa and they could get me a reman ECM. I changed it myself and still nothing. I also swapped the MAP out for a new one.

My next step is a reman distributor. IT never looses fuel or pressure. Almost acts like it drops a cylinder or looses spark. IDEAS???? Please help

[ChevyTech]

It is possible that a worn distributor is causing problems but by the description of symptoms it does not sound like a worn distributor problem to me. On these distributors the housing/upper bushing area gets so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.

It would be a good idea to do a compression test to rule out a lack of compression on one cylinder.

I see it’s your first post. Welcome to the message board.

[CatTech]

I will try that. Thanks for the reply and the welcome.

[CatTech]

Holy COW ! I cant believe I overlooked the egr valve. I unplugged it and plugged the line off the TBI unit and the stumble, puking, missing is gone. Man I thank you so much for the info. I still cant believe after 3 days - $325 of parts ( well includes the tune up parts ) that a simple unplugging of a vacuum line fixed it. Now, can you do anything about the gas prices?? LOL

[ChevyTech]

Leaving the EGR disconnected could cause pinging that could damage the engine, and the service engine soon light will eventually come on, if the EGR is not working.

If disconnecting and plugging the EGR helped, the first step is to identify which type of EGR valve/system your truck has.

Positive back pressure EGR valves - Should have a “P” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Negative pressure EGR valves - Should have an “N” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Port EGR Valves – The EGR solenoid pulses like a mixture control solenoid/fuel injector to control vacuum supply. Port valves have no “P” or “N” and it is blank after the last numbers on the valve.

TBI trucks are generally Negative pressure EGR or Port EGR systems.

This is a site that has a photo and an illustration about EGR vale numbers
http://shbox.com/1/EGR_ID.jpg

EGR valves have a spring in them that can get weak causing the valve to open to much or to easily.

Exhaust system backpressure can greatly effect the operation of the EGR valves on many vehicles. Modifying the exhaust system can effect how well the EGR works if it changes the pressure in the exhaust system. Exhaust back pressure (from a restricted converter) or no back pressure (from no converter or mufflers) can have an effect on how much the EGR opens.

Have you modified the exhaust system recently?

If you find it does need an EGR valve, I would suggest only using a general motors replacement EGR valve. Double check to make sure they sell you the correct EGR valve.

Note: A lean condition caused by another problem can be magnified when the EGR valve opens.

[Boog]

As CTech said. Go back with an AC Delco part and be sure the number on the new part matches the one you are taking off. There is a difference. There are cheaper brands out there but they will not last as long or work as well. And you are better off having the egr operational rather than disconnected.