Still fishing for ideas here. My 85 Z drove all summer long flowlesly, then out of nowhere started boggind and backfiring through the intake. Has not been driven now for a few months as time to mess with it is limited.

Initially thought it was fuel related and probably a clogged filter. R&R filter, nochange. Pressure is good.

Procede to ignition system, spark on all 8 and in good shape. Timing at 10 atdc where it has been the whole time and ran the best.

Compression is good accross all 8 no more than 4 lbs difference.

Lastly it would have be induction related? No clogs to intake.

What's happenning is with everything connected, runs, but backfires in intake which would say it's lean, but the exhaust smells rich as heck. Idles like garbage.

With the TPS (set @.52v) disconnected, exhaust clears out, idles better?

Tap on MAF, no change in RPMs.

 

I presume you're getting no error codes, correct?

Disconnecting the TPS will force the ECM into Open Loop Mode, and operation will be based on the backup calibrations. That can indicate a gross error in one of the sensor inputs. I'd suspect the oxygen sensor first, but it may not hurt to take a quick resistance check of the coolant temperature and intake air temperature sensors to make sure they are reporting something reasonable.

Another possibility is that the EST is being affected. You could reconnect everything (including the TPS) and disconnect the "set timing" connector (EST bypass) and see how it runs.

 

Thanks Vader,

Yes, there are no codes. Cleared the ECM each time I tried eliminating possiblities.

Already went the route of disconecting the EST bypass thinking maybe for some odd reason the dizzy spun. Timing is exactly where it has always been, barely ran but ran, any deviation atdc or btdc, stalls.

The CTS is new (although this dosen't make it immune), had to be replaced at start of summer due to improper cooling fan operation, seems ok, but no a bad idea to double check it.

Didn't know there was a IAT sensor on these, I thought the MAF determined temp by air density accross the meter?

 

sounds like a bad maf to me. have you tried to drive it w/ the maf disconnected?

 

no change.

 

That means your maf is probably broken. I went through the exact same thing with my Z and my old 89 formy. It SHOULD run way crappier with the maf disconected. If there is no change... that suggests that it is junk.

 

swapped with one from running car, same p/n, no dice, problem persists.

 

My 82 would bog and back-fire thru intake during acceleration if I pressed the accelerator past a certian point. I checked everything I could think of. Ign, fuel press, fuel filter, etc was all fine, no codes. Removed the valve cover and started eng. I had a few exhaust valves that would barely open due to the cam lobes being wiped out.
There was a run of bad (soft) cams during that era. Don't remember what years were effected, but it did apply to 305s.

 

Along with that, you might want to make sure the FPR isn't dumping fuel to the plenum.

 

I had the same thing, it was one of the pressed in studs for rockers was pushed up, so that valve was not being opened.
also check your injectors, just one or two bad ones will do the same thing.

 

Cam is good idea, but when it will run the motor sounds normal.

It is definately something in the air fuel mix gone wrong, but it's too cold to be fiddling around outside at the moment. Basic check of the FPR, no fuel in vac line, pressure held 42-44 psi with slow bleed down after shut off so I'm pretty confident that the fuel system is ok, it's just being mismanaged.

With the TPS & MAF on, will barely run and rich as can be.

TPS off, things lean up but runs (probably due to the default values as previously noted.

 

Bad switch for the 9th injector keeping it on?

Have u ohmed all the injectors out yet?

Got a scan tool or something u can use to see the actual reported coolant temp to the ecm?

As mentioned earlier too, older heads with lots of miles can pull studs out.


By the way, in most instances(they are always anomalies) a good maf unplugged will make a car run worse, a bad maf unplugged tends to be better.

later
Jeremy

 

85 Z

check the voltage at the maf sensor. the burn off rely box on the 85's are on top of the ecm. open that box and look for any burnt traces or corrsion.


my old 85 tpi had the same symptoms and it was a burnt trace for the maf power.


Also if the cam is wiped or the studs are pulled out you would notice a difference in compression #'s greater than 4 psi.


get a scan tool on it and check it out.

 

There are always possibilities aren't there?

Unfortunately, at this point not much sluething can be done, it's 14 degrees, car is supposed to be in garage, but with kind thanks to wife who dosen't listen and think ahead some times, car is stuck in the street, already been hit and just going to have to wait, unless it gets towed.

Anyway, I've thought about the cold start injector as well, but again, given the way it can be leaned through default values, I wouldn't think so.

Back to the MAF, I've heard of people running without them all together which would mean it could still be driveable, mine is not, backfires like heck through the intake.

Scanner got up and walked away Whcih it needs to find it's way back because this is getting aggravating without.

Dunno, it's supposed to get back up in the 40 degree range this weekend so hopefully I'll figure something out then and let ya know.

 

I'm liking the idea of the MAF controller. I've heard alot of similar problems with 86 and up that have relays sure been nice if it would have been done that way along.

 

ok, a little update since the weather was a little better this weekend.

Found that the pigtail connector and on the MAF had a good amount of corrosion. Cleaned it up and now things make a little more sense.

Everything connected = starts, idles up normal then falls dead in about 5 seconds.

MAF diconnected = will run and search, going from idle speed down to near stall then back up to idle, over and over.

So I believe I am safe to condemn the MAF?

Racking up quite a debt in parking tickets and need to be certain it's bad, thanks

 

hey dude

Check the voltage @ the connector and also check the burn off module to see if it has burnt any of the traces on the board.

The module for the 85 cars is located on a bracket that sit rite above the ECM under the pass side dash.

 

I understand and appreciate your recomendation, just don't have the time or know how and atleast another half foot of snow forthcoming.

 

you say you tried a working maf? what year tpi was it from? I believe that the 85' TPI used a different maf sensor than later years.

 

Alright, instead of letting a problem thread die without an outcome, here's an update.

Took a gamble on my hunch with the TPS, cleared things up BIG time. Gone is the game of 'if I disconnect this it runs but not good ordeal w/no throttle respnse'.

Now it will start and run. At first still not too good right away. Figured I'd pull it around back and check the timing out. Drove almost flawless to the end of the block, then started crapping out again (bucking, lost power, low rpms)

Finished limping it to the back of the garage.

Little sluething around revealed that in the wiring loom the coolant temp sensor wiring was butchered up and corroded. Splicing of this, things are even better.

It will now start and run, but only with the timing waaaaaay advanced, has throttle response, but after a few minutes of running it seems to go lean out. When you goose the throttle there is a lag before the motor repsonds and if goosed too much it nearly stalls. Will dirve but only above 2500 rpm befiore that it's really choppy. So now I'm back to a fuel delivery problem I hope. Before that tho, does anybody know if the MAF is different for 86 & up tpi versus the 85, I'm with smurfn' and think they are?

 

If you found one bad wire in the loom, I would be looking for another bad wire if you say the timing is waaaay off. Mice can do bad things when it gets cold and need somewhere to get out of the elements. There could even be a gray squirrel taking up refuse in there somewhere. I would be looking at the loom and for any tel tale signs of someone taking up residency in the engine bay.

 

Oh i know that one, I had a squirel fur lined engine bay once when one that was 'taking refuge' in the bay got himself entangled in the serpentine.

My boss thought I feeding him when I told him i was going to be late due to the situation. Needles to say when I got there it was a no contest.

Mice, there's another one. I just 5 years restoring a car because of what they can do.

Anyway, the thought of electrical issue with the fuel pump circuit has crossed my mind this morning. The 'bad wire' in the loom was a result of a prior owner. I am definatley going to check the whole cicruit out when time permits. I have a funny feeling about oil pressure switch and relay.

 

That is a place I would start, the oil pressure switch is about $15 at the part store. I forgot to mention to check all the grouds also. I grew up in Indiana and know that the snow mixed with sand and salt built up around the car from the street plows can reak havoc on a vehicle. Hope you get her going and it doesn't break the bank.

 

I had your exact symptoms, and I shave my head, so I couldn't rip my hair
out without resorting to armpits (ouch-quit it). Mine ended up being a bad
ground for my cooling fan relays. Don't ask me......

I had just finished cleaning out my plenum/throttle body/IAC, and started
it up to burn the chemical vapor out, and my headlights came on by them-
selves along with a right front parking light. By the time I got to the car,
it went away. A few days later it did it again while I was driving, so I jumped
out and popped the hood and started wiggling wires, well wire. I was lucky.
the very first one I grabbed was loose-and when I grabbed it, the fan came
on, lights went out!

I drove home, tightened everything, and re-set my computer-been golden
ever since!!

 

Ready for this?

It was the damn carbon button under the coil was melted causing a grounding situation and weak spark.

That inpart with swaping the MAF to get it to run long enough to actually see the spark go from good to garbage.