I'm still learning, so a lot of troubleshooting is a huge learning process for me. Generally, I read about problems and fool with the car before I come ask you guys, but my problem seems so general that it could be any number of things: rough idle, sometimes stalling while stopped due to fluctuating RPM, using more gas than normal.

About two weeks ago or so, I did general tune-up work on the car to prepare for a 500 mile trip across the mountains and back. Car was running better than ever before in the year I've owned it. Some months back, it threw a code complaining of EGR, which I've found that the sensor has gone bad, and around the same time, had a very rough idle and terrible mileage(no stalling). I cleaned the IAC out and this fixed that problem, so I never fixed the EGR. I've run Seafoam through the gas, oil and intake two weeks ago as well. Spark plugs were changed, oil was changed, etc, etc. Only took me half a tank to get 250 miles on my trip. However, on the way back, it ran like a dog. Took me a full tank to get back on the same route. My distributor is not tightened down(haven't had the chance to get in there and tighten it), so it can vibrate loose. However, changing it's position has not helped the current problem. The IAC appears to be clean, as does the throttle body. The car will start fine and idle around 1000-1500, then drop to 500 after a while. Once it's warmed up to normal operating temperature, the RPM will begin to fluctuate when stopped and sometimes stall. The spark plugs also appear to be fine and not fouled.

My dad had the same car as I do a few years back, and he had a similar problem. The cause of his was fuel injectors going bad. He's also told me a dying distributor could cause the same problem. I've read on the forums that a bad TPS, a bad fuel pump, clogged fuel filter, or any other number of things could cause the problems I have.

So logically, the process for troubleshooting would be to check the main things an engine needs: compression, fuel and spark. The seems to get compression just fine, since it's running. I don't know how to check proper fuel delivery. Checking for spark, as my dad taught me, is to take the spark plug out one by one, still attached to the wire, and start the car and see if a spark occured on the plug, and repeat through all of them. I have not done this yet.

Help, anyone?

 

I would check to see what the voltage of the MAP sensor at idle, 1/2 throttle and wide open throttle. At idle it should be around 1, 1/2=2-3V and WOT should be around 4.5V. theres 3 wires at the MAP : Ground, Signal (thats the one that varies) and Reference (should always be 5V). check that

You can do the same to the TPS if you want. so that way you can narrow things down.

TO check for spark, you can pull out a spark plug and put in the spark plug wire and put the plug close to the block or the head or something like that.

For fuel, you'll need a fuel pressure gauge, and to test the injectors you'll need a injector balance testor, wich I dont know how much that thing cost, but I'm sure Snap-on sells it for alot.

And also, take off you distributor and check all the terminals insde for corossion, clean them with a file or sand paper could help it a little. also check for cracks in the cap.

And tighten your distributor down, it could be moving a little when driving and re-time you car.

And one final thing, I'd look at your fuel filter, If you have clogged fuel filter that would scrue up your car, also crud in your gas tank will do the same and also a clogged catalytic converter.

Good luck

 

If its an 87 2.8, then there is no MAP. What code is it currently throwing?

Definetly tighten the distributor. If you don't know how to set base timing ask.

If you can verify that the spark is good, then try fuel. Get a fuel pressure gauge and hook it up. This will tell you if your pump/filter/regulator is bad.

If you aren't throwing any codes and it still runs crappy, check the MAF! MAF's are terrible creations and can go bad in weird ways. Just unplug it and try to run the car. If its all better, get a new one.

 

I'll check the MAF when I get off in half an hour. I replaced it with a brand new one in September or so. When it was going bad then, the car would randomly die.

It is not throwing any codes at all--that's what's buggin me. I like it when it gives me codes, cause then I have some sort of direction.

I'm really hoping it's not a cat, cause I'll have to take it to a muffler shop...sucker is welded on good; probably the original, though. As far as I've been told, there are no ill effects to removing a cat entirely, and I live in a state with no emissions regulations. Doesn't the O2 sensor plug right into the cat? I'd have to have my chip reprogrammed to ignore the lack of one, right?

 

Starting to sound more like the MAF. I had the exact same problem with a new MAF. Thought I was good, but nope. SOMETIMES, the MAF's go so bad, you can flick it with the car running and have the engine die. o2 is located on the passenger side, right after the pipe y's together. Not on the Cat.

Check my cardomain to see some of the stuff I did, like the EGR delete. Sneaky!

 

I unplugged the MAF and ran about a quarter mile on varying terrain and it ran much more like it should, but kept climbing in RPM. I was idling on flat ground at 25mph/1700rpm. Plugged the MAF back in, and it ran better than it was before with it on, but was still very rough and not quite as good. RPM were back down to 700 idle. Sounding like the MAF, to me, but I'm going to check other things before I go spend $80 on a new one. I'm checking for spark in all cylinders tonight with a buddy, and will check out the fuel filter(about time for a change anyways) when the car cools off.

Assuming it's the MAF, that would mean two of them in the span of a year, which I don't like. Is it possible to convert to something better(SD?) or rid myself of it entirely? Is it worth it?

 

I converted to SD myself. I did that instead of replacing my MAF! Easier to tune and less restrictive design. However, the MAF is far more accurate. Try to replace it with an AC Delco piece instead of the cheap parts store brand if you decide to keep it. Before you buy a new one, you may want to try to swap with someone with a known good one.

If you want to do the swap, grab a 3.1 harness and computer from a 91-92 3rd gen. Its really not hard to do the swap when you do it like that. I made an adapter harness and added the MAP sensor. Still working out bugs....

 

IAC is already clean. That was the problem last time, so I figured it was the problem this time, so I checked that first thing.

I don't know anyone with another thirdgen, so I wonder if a parts store will let me hook up one of theirs to test out? Probably not. :/

 

I'd realy try to thighten your distributor though

 

Aye, I intend to. My only available time is night, and my roommate's car is currently sitting on the cord going to my drop light, and the car doesn't have a real axle on it currently... so it may be a bit.

 

So I finally got the distributor adjusted and tightened down. Car runs much better. I had my hood off to get it done, and was driving around my neighborhood jumping in and out of the car to adjust it, so my neighbors had a good chuckle.

However, with the hood off, I was able to hear that I've got a vaccum leak somewhere when I'm at half throttle and gets worse as I give it more. I can't tell where it's coming from at all. All the threads I've read about vaccum leaks on here simply say "find it", "get a gauge", and "plug it". What's a proper method of going about testing all the vaccum lines? Where do you start? I understand that vaccum is pulling air into the engine, so the logical place to start would be at any lines are directly on the engine and work your way outward, right? Is there a vaccum line routing diagram anywhere?

EDIT: I went out and bought a vaccum meter and hooked it up inline behind the intake on the smaller hose on the passenger side, then took her for a drive. At idle, vaccum is 18. During varying driving conditions, vaccum went between 17 and 20(low and high) with an average of 18.5-19. What level should the v6 be running at?