I just took the truck out for a drive, and i thought i had solved it for a while, but apparently not. My engine always has had what seems to be a random intermittent miss at an idle, but recently it had been getting really bad, and I thought it was my ignition wires which were to blame. It got to the point where just idling on the drive, it would start to lope a little and then even stall some time. I looked and looked and found no air leaks, and my vacuum gauge looks good. I messed with the timing, and it runs a little smoother when its over-retarded, but thats not solving anything. I unplugged my O2 sensor, and no change in the engine was heard. Finally I unplugged my MAP sensor, and everything seemed to smooth out. (I left the vacuum line intact). I was wondering if anyone knew what was happening here. There were no codes, but is my MAP sensor bad? Or am I just treating the symptoms by eliminating it??

 

Do you have a manual? Do you have a volt meter? You are going to actually have to do some diagnostic tests to determine what is wrong. There are voltage and resistance checks that can be done on all of the computer input modules. (MAP, TPS, temp sensor, O2 sensor, and Hall Effect switch) No one can trouble shoot a vehicle accurately without doing these tests. We can help you interpret the info you get from these tests but you must do them or we will just be shooting in the dark.

 

Ya, I have the books.... once I am through with my exams this week I will start doing some more in depth diagnostics.... will an analog meter do the trick? My digital is at my parents place.

 

An analog will do all the tests. Some of the tests actually require an analog meter instead of a digital. I look forward to helping you with more info.

 

Thats awesome, thanks to everyone for advice and guidance.... I will report back once I have a little more information.

 

Alright, so I took a study break.... I couldn't help myself, the truck was just sitting there.

So I checked 2 of the sensors, I did a reading on the MAP and the TPS. Both of them were perfect to spec. Just <5 volts on both, and the MAP increased slightly as the vacuum dropped, and the TPS increased linearly as I turned the dial between 0 and 5 volts. At the same time I reset the computer just to clear anything left over. On top of that, my water temp sensor is brand new. I have lots of extra parts from the scrap yard, so I have tried swapping out the idle adjuster servo, injectors, TPS, and I also cleaned up the hall effects switch just to make sure it was working. Whatever is going on feels like a miss. Its as if every 5th revolution the spark just stops for s second. Oh, and I have swapped out the coil as well. Nothing has worked. I have been having the same problem for so long, and it seems to be similar to many other people I have read on here, where I will be sitting at a stop light, and the engine starts to lope, and almost stall. It also doesnt help that my tranny pulls pretty hard in gear, so it makes it even worse when I am at a light. My exhaust smells a little, but no color, no other codes apart from the rich mixture, new plugs, wires, thermostat etc... I am totally stumped on this one

 

What was the specific output voltage from the MAP with no vacuum applied and what was it as you increased vacuum to the MAP with the external vacuum source? What dial did you turn when checking the TPS? What was the output voltage of the TPS at rest? What did it go to at wide open throttle? If you want help I need to be able to interpret specific data. Cleaning the Hall Effect Switch does not insure that it is working correctly.

If I was worried about a truck that does not run correctly. I would first hook up all of the systems that came with the vehicle that have been disconnected to eliminate that as the problem. These systems were all put on that vehicle for a reason and in my experience problems do arise when they are disconnected.

 

The TPS (when removed is no longer held in idle position) reads 0 volts. I used a screwdriver to turn the inner circular section where the throttle shaft goes in. It ranged evenly from 0 to just under 5 volts as I turned it. The Map sensor reads 5 volts when engine is off. I then started the engine, and watched it drop to about 1.5 volts. I then revved up the engine to drop the pressure, and I watched the needle rise accordingly to about 2 volts or so. I do not have a hand pump, but these values checked out alright as far as the manual goes. I have yet to preform tests on the O2 sensor, but that is a little more in depth.

This morning I drove to school, and the engine sounded really good, hardly any loping, and drove well, then as I pulled into my driveway, it stalled.... uggg. If I had a bit more money I would just go out and replace the O2 sensor right now. As for the hall effects, I would say thats not it, simply because I have had HE issues in the past and they were totally different. When the HE was bad, it would stall at highway speeds, mine is mostly only noticeable at an idle.

Just a thought, I read someone on another site who said they solved a loping idle by adjusting the idle on the TB, mind you it was a GM or something, but they said it had something to do with the fact that the idle was adjusted too high, and causing issues keeping a steady idle..... is this a possibility? I know there is no idle screw, but there is a min idle adjustment.... I wasnt sure if this could cause loping.... but I guess that wouldnt cause a rich mixture code, although they COULD be unrelated ailments.

 

A low idle could cause the engine to lope and stall but the actual idle cannot be adjusted within the system on our trucks. If the idle is too low the cause must be corrected. An accurate tach is the only way to tell what the idle actually is.

The MAP sensor should read 5 volts with the engine off as yours does. At 5 inches vacuum it should read about 3.75 and as manifold vacuum increases to 20 it should drop to about 1.5. This is hard to do with the engine as a vacuum source but these parameters must be met to do an accurate test. Movement in the proper direction is not good enough to assume the system is working correctly.

The TPS cannot be correctly checked off the engine. It must be checked with the engine off, throttle at idle and then use the throttle to go to wide open. These are the actual limits that the computer sees and reacts to and they must be within specs with a smooth change as the throttle is opened.

I learned a long time ago that if I could not find a problem, I could not assume that any system was working correctly until it was checked. If you eliminate any system as the cause of the problem without actually checking it you may never find the problem.

 

I have a tach installed on the dash.... my idle is around 800 - 850, which is good. It idles a little high during warm up, and then usually drops back a bit. My hard pulling tranny brings the idle down to about 550 when in gear though, and it brings the vacuum from 20 down to about 16, so any misfire of imperfection is made 10 times worse by this. I will recheck the TPS attached to the truck, but I have swapped out that part 2 or 3 times, and none of them have solved anything...when I did take it off to check the response, it DID throw a code for voltage being too high or low, so at least I know the voltage is within spec for the computer when it is attached. Im still skeptical about the MAP and the O2 though, so I will look into that further. Unfortunately these are the 2 most expensive parts...107 bucks for the map, and 80 for the O2.... I would like to rule them out if I can.

Im gunna go down to parts source when I have some time and see if I can borrow the vacuum tester.... they have the tool lending program there, so I should be able to rule out the MAP sensor.