Injector problem? Bad CATs? O2 sensors? If it seems to be running well but is backfiring & smells of gas then it seems like you're running rich and/or your CATs aren't burning off the excess gas properly (there's almost always some unburned gas). I don't see any mention of a scanner in this thread but one might be handy here. Just guessing without more information.

 

It doesn't run perfectly, power is back to maybe 70% of its original level. Furthermore even though the idle is now stable, the car still vibrates a bit too much and feels like there's a misfire (it doesn't "purr" like a V8 should).
I'm starting to wonder if one of the new spark plugs might not still be bad, maybe I should try looking at that first ?

CATs have always had a problem I think, even when the car was running fine there was a raw metal against metal sound coming from the CATS under very hard accelerations (turned the SES light on). It obviously restricted the flow of exhaust since the car was had less power and felt "held back".

 

Looking at the plugs again might tell you if a particular cylinder is fouling (due to injector problem?).

The cats are there to burn off uncombusted fuel & the O2 sensors are there to check on the cats & provide feed back to the computer.

It's all connected.

 

here is a good idea to bring some order into caous. using tight plastic rapes on wires. I will post couple of pics. but I think I will stard a new post with is information since I think is so helpfull[IMG][/IMG]
[IMG][/IMG]

 

plugs can show evidence. it is possible that one was cracked while installing, is easy to do if you don't use a universal joint. or the gap can be changed. all things are possible.

have you noticed rpms bouncing on tach, or engine running cold? everything is connected, as the man said. only the OP knows what the history is. was the dist rebuilt, or am I on another thread?

 

I've never actually changed plugs so I'm asking; what's the story with the universal joint ?
Rpms bounce from 6 to 7 or 7 to 8 when idling, but I never considered it as a problem ?
Outside temperature is really high these days so it's hard to say, all I know is after a 10 miles the car was still at about 60°C. (when leaving it idling when I did the timing light job it did reach 100°C though).

 

u joint, "swivel", whatever, allows the socket and extension to "bend" and flex in case it is hitting a/c bracket, whatever. so that the socket is seated squarely on the plug.
as for plug gap wrong, it is not likely that one was dropped, but has happened, and gives symptoms of misfiring. the point is that all things are suspect until verified.

60*c is low, 140*F, for an engine on the road.

tach bounce of 100 rpm is normal, I think, was wondering if the tach bounced with no evidence of engine increasing. points to dist, wiring, etc. the spark and timing start at the dist, with pickup coil, also reference pulses to ecm

just following some fsm possibilities, suggestions when nothing seems obvious. checking the plugs -not my fav job- is the one way to get a clue. fsm says that coil and pickup coil polarity wrong will/can affect performance. but...recently rebuilt dist on 91, changed out original module and pickup, and pickup leads were "wrong" per fsm, but had no performance issues at all.

 

Ok yes I see what you mean by swivel. Apparently driver-side sparks are easy to reach, but the other ones...is there something I should know about how much torque to apply when putting them back (30 nm according to SM?), or general spark plug related tips ? I wouldn't want to break one or not tighten it enough.

I don't have any significant tach bounce so we can cross that out.

To sum up, so far, the symtoms are as is:
- the car now starts normally, no more need to accelerate, no more stalling upon cold starts,
- timing has been set right following distributor cap +rotor replacement (was at 4* retarded, now at 6-8* advanced); however this now produces massive cat explosions,
- I definitely think that there is a misfire, as the engine doesn't purr like it should, and when I start from a dead stop if stutters and vibrates. Acceleration is back to 70% of what it should be but noise is still not what it should
- mileage is down, and the car reeks of unburned gasoline.

So I'm thinking:
- bad spark plug, or
- bad injector ?

 

on the 91 I have to remove inner fender, some 15 torx heads, #15 and 20 I think, to access #2, 4. torque in alum hds is 19, less is more.
I use a couple drops anti seize but GM says avoid it, use one drop oil. more is less here, because it all is pushed to the gasket anyway.

are you positive that timing is correct? ex, top of timing tab scale is advanced, from zero, btm half is ADC, after, ie retarded. stranger things have happened!

IF the explosions started precisely the same time as timing change...

from the picture, it appears that your dist is advanced about what I'd expect, though, assuming it was installed with #1 dist pin pointing at 5 on the clock.

never rule out injectors, even new Bosch's, from what I've read, although mine are fine. but, if engine ever runs WOT with no miss, I would not suspect injectors at low end, nor explosions in exhaust.

stutter and vibrate means cylinder isn't firing, usually, or not on time. watch your timing mark in park with all systems, EST, connected normal, to see if it ever retards lower than zero as you open and close throttle. ecm retards spark on signals from knock sensor, and others.
I recall someone suggesting to unplug EST and watch for a reaction on the road test. would prevent ecm from advancing or retaring spark.

if you have fsm, there is a simple -nothing is simple on a c4- injector harness test to see if all injectors are firing. have to check each one.

all these cks are free except for time. as the book says, it can be ANYTHING. a bit of exaggeration, but you get the message.

saludos,
joe

 

That's exactly what I thought when I heard the result; just to be sure, the mark has to be above the 0, right ?

Yup, that's right, cylinder #1 is at 5.

It doesn't run WOT without any misses, there's still the distinctive misfire sound at every speed...

That's a great idea, I'll try that next time !

I don't have fsm, do you mind explaining the test ?
I checked the SM, they suggested also checking for:
- air leaks at air duct between MAF and throttle body,
- binding/sticking TPS,
- EGR not opening
- fuel pressure
- leaky valve oil seals (ouch!)
- output voltage of ignition coil...

 

spark BTDC is above the "0", yes.

the inj harness test is to unplug all injectors. push the spring in to release, put a test light in each plug terminal- one lead in each terminal of the connector. the light should light while cranking.

if one harness connector is not firing, the engine will miss, but that would not explain the noise in the cat, because no fuel was injected into manifold. the "symptom" of miss/cat explosion is a result of gas charge not ignited in cylinder, way I am thinking.

faulty pickup or ign coil will cause poor performance, says the fsm. GM uses a spark tetser to test for ouput voltage, 25,000 v min. should be used in case if ign miss, because the system may not provide enuff v to fire plus under load.

if possible, borrow an FSM. the charts for checking ign faults are too detailed for a forum. if you are inside the dist again, check for the color of pickup coil leads and ign col leads.

I'm still thinking the spark plug check will show something.

saludos

 

Agent_86 has a stash of '86 FSM scans...see post 6 of this thread:

http://forums.corvetteforum.com/c4-g...ic-inside.html

 

Thanks for the link
From what I see the FSM is what I already have (yay !)

I'll do what you suggest Joe, and keep you informed. I sure hope it's something as simple as a spark plug...
Just to know, if I do find a foul spark, should I just replace it (seeing as though they have just been changed by the shop...) or investigate what fouled it ?

Also, can anyone tell me what round piece at the center of this pic is ? (the one with a tube going in an out), because mine leads to nothing.
Thanks !
http://i1111.photobucket.com/albums/...9-29155125.jpg

 

change plugs and also investigate. Duh! take a picture if not sure.

the round pc -from hitmanpty I believe- is a sort of vac accumulator, collecting hoses that supply under dash, maybe gas fume accumulator -can't reall the proper names. it is there for a good purpose and if there is nothing attached, it would falll out, no?

I say it cannot be ignored, for one there may be vac lines open to manifold. for two, with the problem you have, it can be related.

 

It is indeed hitmanpty's pic
It's because I had started looking for vacuum leaks at a time and found this partly unplugged (see pic below I took), but I don't know for how long it's been like that. It's connected on one end but as you can see the upper side (next to the distributor) is connected to nothing !

While looking around I found out the oil pressure switch (the one which turns on the light) was disconnected, even though the mecanic had supposedly changed it, so I put it back on...

 

(don't believe it was his car, but his pic)

No, I can't se where the end goes, appears to be the plastic line that leads to my MAP sensor. 87 has maf, but it goes to a vac source. there is an open vac hose under the pink bat wire, no? that goes to an outlet, not to engine vac, to doors whatever under dash.
my 91 ha a tee on one end, whre you have the elbow, 2 vac hoses, another hose goes to a tee that serves gas recovery and cruise. all those things are needed, also, the plastic line goes to vac source, I believe. "leak."
just something else to do.

I would be surprised if your mechanic did anything at all.

 

I dont even know and is my car. now next time a I will have to check this to see if all the connections are ok

 

looneytune, the vac source is the plenum, a brass tube on right bank rr top, with another that feeds FP reg. I said mine went to map but meant behind map, since map is electrical. surprised someone didn't remind me already! *** call it another senor moment! the MAP also has a vac line to sense PRESSURE! the key word. three lines at that point into plenum. I recently used a meter to ck MAP sensor and got my brain out of sync.

I doubt that you have an open hole in your plenum, but it is possible. maybe the plastic line is pinched or melted shut for some -stupid- reason.

saludos. (don't believe it is your current problem but "in addition to.")

 

Oh okay, I see what you mean. It's probably been there forever but still, I'm trying to get everything right

 

I agree, If my msg was not clear enuff, the terminus of that vac tube is the plenum, so if that is not capped shut, it is a leak. if it is or not, it needs to be put back original. it can upset the fuel/air ratio, but not sure how it can cause rumblings in the bowels of the cats.

joe