95 engine issues

[metallica21156]

well i've been having issues with my motor. so i got a scanner and now i can look at what the motor is doing. its opening the egr valve up under a load and the motor looses power. the computer is sensing a knock. anywhere from 143 to as much as 256. the only other thing ive seen is the timing. at idle when its in closed loop its aroun 11-16 advanced. if i give it 7% throttle while power brakeing it, it goes to around 35% and then it senses a knock and opens the egr. it does this while driving to. new map sensor and throttle postion sensor. removed egr today and cleaned just to make sure. any ideas?
its a 95 witht he cpi. and i changed the regulator about 3 weeks ago. this has been going on for a long time.

[old_master]

Low fuel pressure causes a lean mixture which will cause a spark knock. Which injection system do you have? 95 came with CMFI or CSFI. Have you checked fuel pressure and leakdown?

[metallica21156]

i've checked fuel pressure and leak down. i've even driven the truck while watching the pressure and it never feel below 65psi. it has the cpi spider.

[metallica21156]

can anyone tell me what the timing should be?

[Leeann94astro]

I believe it's 0° ATDC; the computer does the rest.

[metallica21156]

i need to know what it should read on the scanner.

[metallica21156]

well the latest update is that i replaced the knock sensors and now it detects no spark knock. but im still having the same issue of the harder i accelerate the more the egr opens unless i floor it. then it stays closed. basically its operating opposite of how it should. what controls this thing?

[metallica21156]

no ideas here?

[old_master]

EGR should be closed at idle and at wide open throttle. It will vary inbetween there as the ECM sees fit. Sounds like its working correctly.

Fuel pressure at 65psi while the engine is running is too high. With the key on, engine off, fuel pump running, fuel pressure should be 58psi to 64psi on your 95 cmfi system. The pressure must remain above 53psi for 3 to 5 minutes after the fuel pump shuts off. GM does not publish any "engine running " fuel pressure specs, there are far too many variables involved. Typically that system will maintain approximately 53psi at idle, and increase up to whatever the engine off, pump running reading was, as engine load increases.

I see in your signature that you have a K&N intake They usually cause more trouble than they're worth... Oil residue from the filter will coat the MAF sensor causing it to send a false reading to the PCM. Have you cleaned the sensor lately just to be sure?

[metallica21156]

it would hold 65 while driving but it did change depending on engine load. at idle it was 58 to 59 if i recall. the truck runs great with the egr unplugged so i dont believe it has to do with fuel or spark. its whatever is controlling the egr. i know the egr is supposed to be open but its work backwards. the more load that is put on the engine the more it opens. less load the more it closes. my parents 98 closes once throttle goes past 30% or something like that. mine just opens more. the intake is a cpi system so there is no maf. and i've replaced the map sensor with no change. i need to know what else controls the egr operation. i've also tried switching ecm's with no luck.

[old_master]

What is the fuel pressure with the key on, engine off, fuel pump running? 65psi is more than that system should EVER produce. If your gauge is accurate, I would suspect a restriction in the fuel return line.

When the ECM is in open loop operation, EGR does not open. When the ECM is in closed loop operation, at idle the EGR valve is closed. As engine RPM and load increases, the ECM opens the EGR valve unless/until WOT ocurrs, then it closes. EGR reduces combustion chamber temperature which reduces detonation, (spark knock). It also reduces NOx, (worse than CO because NOx consumes oxygen.) The ECM has total control over EGR operation and commands it according to data received from various sensors on the engine.

[manicmechanix]

I could be wrong but I think your EGR now sounds like it's working properly if it closes near full throttle. I don't think you can compare a 98 to a 95's operation. They are different systems. I think your timing sounds about right. Does it run OK?

[metallica21156]

as far as the fuel pressure goes when i checked it, i was at a friends shop. we pulled up on alldata what the specs were and testing and all the pressure read correctly. i cant see anything being wrong with fuel delivery if the truck runs perfectly with the egr unplugged. as for the egr, it follows 100% what the computer tells it to do. i just need to find out is telling the computer to open it. i did get another code the other day that said o2 sensor in bank 1 sensor 2 had low voltage. i reset it and hasn't came back since. its the o2 after the cat. would this sensor effect the egr? with the egr pluged in when you hit the gas to take off it stumbles and then recovers and goes but it feels like your running on 4cyl. unplugged it feels like a v8.

[old_master]

The O2 sensor after the converter monitors catalyst performance ONLY, that's all it does. It has nothing to do with fuel mixture and it does not affect EGR.

I'll do some research on what the parameters are for EGR operation... should be able to get back with an answer tomorrow.

As for Alldata specs on the fuel pressure, I would trust the GM factory shop manual.... 58psi to 64psi. If Alldata is giving you an "engine running" pressure spec, don't believe it, there is none. There are far too many variables involved that affect fuel pressure while the engine is running such as basic engine condition, engine load, percentage of throttle opening, altitude above sea level, ambient temperature, coolant temperature, etc.

[manicmechanix]

Well if it still runs a lot better with the EGR unplugged there might be a problem with the EGR. I don't know what it could be. I think the EGR command is based on the input of several sensor most of them you replaced or aren't throwing codes such as the VSS. Maybe check the wiring harness or maybe the EGR valve is wacked and tricking the ECM/PCM .