rogan

iTrader: (7)

I recently purchased a '91 na completely stock. Warm Compression on one rotor is 95-95-95, second rotor is 95,100,110. It had several gremlins, I am chasing down. Fixed a couple of small vacuum leaks and I have the TPS adjusted in. There are no engine codes. Idles perfect and smooth at 750-800 rpm.
Now as I rev it, it starts backfiring. The higher I rev the worse it gets. Small pops at 1500 to giant bangs at 4000.
I have verified that the timing is advancing as the engine revs. TPS is adjusted in. I have spent hours searching for vac leaks found and fixed 2 very small ones that ended up making about 300 rpm difference at idle that I corrected with the idle screw.
New stock plugs and wires.
Any ideas? I am thinking about fuel pressure next, but it pulls hard under load, doesn't act like starvation.

satch

iTrader: (1)

Since your TPS was wired wrong the fixing of this problem could possibly stop the popping.

spectre6000

iTrader: (1)

My first reaction is to make sure the plug wires aren't arcing. The spark voltage can jump out through the insulation and cause two plugs to fire weakly at the same time versus one firing at the appropriate strength. It's as simple as putting some space between the wires (zip ties) and checking to see if it still happens.

If that's not it, check your coils.

rogan

iTrader: (7)

Satch You must have me confused with someone else. My TPS needed slight adjustment, what do you mean wired wrong?
Spectre6000 I have put a new set of stock plugs and wires on it, no change. I checked the coils and even swapped them with a set from a S4.

satch

iTrader: (1)

I indeed did confuse you w/another poster.

spectre6000

iTrader: (1)

New or old makes no difference on the wire arcing. Some insulation is just better than others. Absent higher quality wires, separating what you have with zip ties is usually all it takes.

monty11ez

What method did you use to adjust your TPS? I find the JohnR/Factory Method to work the best. 2nd Gen TPS Adjustment

wozzoom

If your TPS is close and passes the factory service manual checks, then I would look elsewhere for the problem. A bad AFM will cause backfiring. I'd also suspect the fuel injectors and lastly the engine CPU.

What color are the spark plugs when you pull them out: Black and wet, or a light tan color?

Have you made sure all the engine grounds are clean?

rogan

iTrader: (7)

For TPS, I used the fsm method first measuring the resistance and had trouble getting it in spec. i then found the voltage method, and I adjusted it per that with no problem. I just hated to pierce the wire insulation with my probe to do the voltage method, but it is easier.

rogan

iTrader: (7)

The old plugs were mostly dark brown and grey, no real build up, no wetness. I cleaned the ground on the upper intake manifold and the one under the trailing coil. The manifold ground was pristine, the one under the trailing coil, had a rusted bolt. I did not have another bolt, so I cleaned it up with a wire wheel and chased the hole out with a tap.

rogan

iTrader: (7)

I will pull the AFM and check its resistance readings against the FSM.

wozzoom

Check the AFM resistance. It won't 100% confirm a good/bad AFM, but its something ruled out. Just to be clear, does the backfiring occur when reving the engine or when under load? Or both?

satch

iTrader: (1)

You backprobe the TPS wire (Green/Red wire) at the connector plug as there is no need to pierce the wire itself. Also, the car needs to be driven at least 20 minutes to sufficiently warm up the engine that's necessary before setting the TPS. You also need an analog meter to test the sweep of the TPS to see if there are any spikes. It should range from 1 volt at idle to close to 4.5 volts when fully depressed.

And does the backfiring occur while lifting off of the gas pedal (decelerating). If so, there is a an anti afterburn valve in the ACV.

rogan

iTrader: (7)

Maybe your TPS is different, but on my '91 you can not back probe the connector. The connector has hard plastic grommets around the wire in the connector, it is solid. you can not stick your probe in the backside like most other connectors on the car. The only choice is to pierce the wire for the narrow and full data lines.
The backfiring occurs when holding the gas steady or accelerating. Anything over about 1500 rpm. Load or no load. It actually stops when you take your foot off the gas.
I have to finish a water pump, timing belt, valve adjust job on my beater car, it is my commuter for work. I will get back on the 7 in a few days and check the AFM.

rx7racerca

iTrader: (1)

It's probably a stupid question, but are you sure you didn't misroute a plug wire (or two)? Sounds to me like lead or trail on one rotor swapped for the other - or maybe just lead and trail swapped on the same rotor(s)? The last seems more likely, as revs increase, the amount timing is out would increase. L for Lead or Lower, T for Top/Trail is how I remember.

monty11ez

I had this happen to me. The PO swapped the spark plug wires. It was an easy fix.

rogan

iTrader: (7)

Yes, I know the leading is on the bottom. I verified the wires before I changed them, i had the same idea about the previous owner, but he had them correct. It was a good suggestion though. I have had the wires on and off a couple of times verifying the coils and I know they are correct.

satch

iTrader: (1)

In all of this have you verified spark at both coils?

rogan

iTrader: (7)

Yes, back when I checked my timing, I pulled the wires off the coils, one at a time and verified good arcing.
I pulled the AFM off today and the resistances all checked out per the FSM.
I guess maybe the fuel injectors are next.

spectre6000

iTrader: (1)

I would be focusing on ignition more than fuel... Especially with injectors as pricey as they are. Gasoline engines (versus diesels, say) require spark to ignite fuel (or hot spots or a mismatch between high compression and low octane). Additionally, there are different kinds of backfires; intake, exhaust, and internal. With a fuel injected engine, you're not likely to get an intake backfire, and the backfiring you describe does not sound like an exhaust backfire. That leaves internal, and unless you have some pretty serious problems in your keg, it's not going to be a hot spot. Bad gas could be an issue, but not super likely.

That leaves a spark issue. The plugs are sparking at the wrong time. If your coils, wires, and plugs are all properly affixed according to their individual idioms, that leads you up the chain of command. What is causing the plugs to fire at the wrong time? It could be something in the ECU, but more likely it's some sensor that's feeding it bad information. Pull back the PS carpet and get to the ECU. Probe the whole shebang and see what's telling you it's unhappy and report back.

rogan

iTrader: (7)

I probed the ECU and everything checked out per the FSM.
I decided to play around with the ACV. With the air pump hose off, I get suction on the line when I rev it to 3k and let off for the anti-afterburn valve like normal. But at idle I have lite air leakage out of the line. When the car is backfiring, I can feel the air leakage pulsing in sync with the backfire through the air inlet port.

rogan

iTrader: (7)

I have made a new discovery, Do to the backfire, I had not really reved the engine past 4000rpm. I just found out that the backfire stops at 4500 rpm. So it starts small at just over 1000rpm to really bad at 4000rpm and then completely stops at 4500 to 5500 rpm. I did not rev past 5500 rpm. This is without load.

monty11ez

You might need to get your primary injectors cleaned.

rogan

iTrader: (7)

Unlike most, I decided to post what the end result was....................
Eventhough the TPS passed the requirement in the FSM, it was bad. After verifying the primary injectors were good, I circled back to the TPS. After looking at many used ones for sale in the forum and on ebay, I decided it wasn't worth getting someone elses problem. You do not even want to know what S5 TPS sell for new now. I checked every local parts chain, and spent hours searching online for the best deal. There is the factory one and two aftermarket ones available. One of the aftermarket ones is who actually supplies the stock one for Mazda. Prices vary well over $200 depending on where you go. Even everybody's darling rockauto gets $565 for one. I finally found a place that had new old stock and got the their last one for $290 shipped. If anybody finds them cheaper let me know, I will buy them up and resell them on ebay in a couple years for double price. There are actually listings on ebay for new ones at $700.
Edit.....I just discovered amazon now has them for $300 with a 4 to 6 week lead time, for anyone looking.

clokker

iTrader: (2)

Congratulations on solving your problem and thanks for posting final results.
Gotta say though, if faced with a $500 TPS replacement cost, my next step would be an aftermarket ECU that ditches the stock part (and the AFM) for good.