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I have a 1990 chevy k1500 4x4 with a automatic 4.3L V6 motor. Recently I have been throwing money into it left and right. It started with a leaky intake manifold,which I fixed. When doing so I forgot to mark the distributor cap but everything else seemed to go back together correctly. I have taken it into a shop to have it timed everything went ok. They noticed my PVC valve was not venting properly, so I pulled off the throttle body and found the port for the PVC was completly blocked with oil build up, I cleaned the entire throttle body replaced mounting gasket,PVC,removed valve covers and removed all condensation and steam build up from the top of the inside of the valve covers,ran a oil cleaning additive then did a complete oil change. I got it all back together and about a week later the idle is all over the place. I took it back to the same shop and they said they could not find anything wrong. I get it back and it still idles incorrectly. About two days later it finally reads a service engine soon code which were 22 (TPS sensor) and 33 (MAP sensor). I had both tested, the TPS was reading a little different than a new one so I replaced it nothing changes,but trouble code 22 no longer appears and 33 still does. The MAP sensor tested OK but in the end tried new one as well, nothing changed and still reading trouble code 33 I've checked all vacuum lines and electrical conections everything seems to be ok. I have tried replacing the Idle Air Control valve as well nothing changed. I have tried replacing the ECM nothing changed. When doing so I tried using a ECM from a 89 2WD chevy with same motor but it was manual the service numbers were different but called autozone and they said both go for my truck so changed the PROM's form one to other installed and truck ran better for the most part but read out service engine soon code 51 incorrect or faulty PROM, found out buy calling around that the ECM is for the same truck but with different options,so i figured this one makes it better so it must be the ECM well I bought the correct one for it and nothing changed. I have tried spraying carb cleaner around intake and vacuum lines and idle does not change. I have checked to make sure I have decent presure comming from exhuast to try and rule out catilic conveter, and replaced oxygen sensor still nothing. The distributor cap and rotor along with wires and plugs have all been changed within the last 8 months, but I checked for loose plugs and wires ,pulled the plugs noticed each one had a litlle dirty burnt looking spot so I cleaned them checked gap seemed to be fine so put them back in. When it starts it idles OK for about 2-3 minutes then either starts idling all over or dies. When put into gear it often dies or idles worse. When driving it feels sluggish and lacking power when I come to stops the idle drops along with oil pressure and voltage.When put back into park after driving idle often shoots way up. Ive tried just about everything I can think of and I think when it comes down to it, the problem lies somewhere between the MAP sensor and the engine running as if it were under heavy load by advancing my spark and changing fuel consumption.PLEASE HELP if you can I am a college student with no time or money to be dealing with these problems and classes start back up in about a week I would like to hopfully have it fixed or at least figured out before then so I can have my head cleared for learning. Thanks for you time sorry it was so long and drawn out but its says to be specific as possible. I dont know how to make this say it can be posted on the help web site but it CAN specailly if it helps someone else with same or similar problems.
Answer -
Hello Mike,
It is long, but very well spoken.
I didn't see anything about testing the vacuum, and cleaning the EGR valve.
It is something that sometimes needs cleaning. It will close on a piece of carbon, and then it is a big vacuum leak.
I assume they disconnected the electronic spark timing bypass connector when they timed it.
Connect a vacuum gauge and see if it stays normal.
Van
Im not sure how they timed it but it was a well known shop in this area (not a dealer shop though)I read that if Im getting a trouble code 33 MAP sensor to check and make sure the voltage going to the sensor is correct , so I checked. Normal is 5.0 volts when I pluged into the MAP sensor connector then turned key to on position the meter read 5.84 and was rising after about two minutes of letting it just sit the meter reach 6.04 and was still on the rise so I unpluged and now Im here and it doesnt tell you what to do if reading is off (but remember I already have new ECM) I was thinking of unplugging vacuum from throttle body to MAP sensor and then plug off the throttle body end and still vacuum from somewhere else to make sure Im getting proper vacuum (do you think this is a good ideal if so where should I still the vacuum from
AnswerWhat the MAP sensor reads is intake manifold vacuum. (Manifold Absolute Pressure).
It could be connected to any manifold vacuum source.
I really don't know if the voltage rise is critical or not. it uses a 5 volt refference signal, and then returns a value, according to vacuum, but I am not sure what happens if the refference signal increases, since it just returns a value of what was sent.
So you are getting beyond my personal experience with that info.
You could disconnect the exhaust system in front of the cat and go for a ride, to see if pressure is building up in the exhaust.
I had some trucks several years ago that had screens inside the tailpipe...about six inches in from the end. The trucks would idle good, but if you went down the road and poured the coals to it, it would loose power, etc. The clue was the hissing that I heard from the exhaust connections.
Seems big rust flakes were stacking up against the inside of the screen when running hard, but would fall off when the load and rpm was removed.
Knocked the screens out, and ran like a new truck.
Four of the same type trucks. 85 K3500 1 Ton, 350 V8, standard.
Worth a try.
Van
