I am getting at a loss on this one.
It is an 87 duke (in an 84) here is the problem, and it did not start after the swap but a few months after.
it backfires under load, to explain, 1st gear, from idle (1000rpm +- 100) stab the gas it backfires out of the intake, 1500 RPM stab the gas no problem,
2nd-5th gear bogged down driving, backfires like crazy.
what I have done so far, I have lots of parts so swaping out parts to test is free, just to supply info.

Swaped to test:
ICM, tested at Autozone, both org and replacement pass after 10 or so tests. and both have the sheild.
Crank Postion Sensor
Coils, both org and replacement pass ohm tests on pri and sec coils
Fuel pressure regulator (was leaking, no change)
MAP, both read the same under the scaner
EGR, both org and replacement pass tests
ECMs, one with the most updated code

Tested:
Plugs look good(perfect), replaced mabey 2 months ago, gap at .060 AC Delco
Compression test, about 155 all around
Added air to each cyl, no bubbles out of the coolent, so good compression and no blown head gasket
spark plug wires are low ohm(high pro wires) and test like 200 ohms for the whole wire, no arcing, no crossfire
No vac leaks found
injector has good spray pattern
spark under load
Scaned the ecm all sensors look good, no flat spots in the TPS, coolent/Air temp sensors seem to read right, MAP seems OK too

Haven't tested,
fuel pressure

Now here is the weird thing, I had it running good, no backfire, drove it all good, shut it off, went "oh the windows are down" started it, idle went to 2500 when the idle droped, idle rough and the same problem returned. Thought mabey the ECM learned something bad, unpluged the ECM, pluged it back in same problem, Thought "mabey not long enough to reset it" unpluged it, pluged the first ECM, same thing idle to 2500 and the back fire when it dropped and stab the gas.

Any ideas?????

And no Orge I don't want links to the cave, I read it all, good info, no help.
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"DRIVE IT LIKE A FIERO"
'84 Fiero, engine to be determined '87 Duke (Sold)
'87 Quad 4 H.O.
'87Blue GT 3.4L Swap Completed!!!!!!!! Boosted!!!!!!!
^^^^ Now in the Construction Zone^^^^
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[This message has been edited by sardonyx247 (edited 02-16-2015).]

Test fuel pressure next.

[This message has been edited by jaskispyder (edited 02-16-2015).]

Spark Test using normal normal plug often lies.
Use st-125 or other tools that emulate real load. (25,000 volts) They give more reliable test.

"Coils, both org and replacement pass ohm tests on pri and sec coils"
Might be ok but Ohm test often are useless because can be temp sensitive. You can pass test w/ normal DVM and pass yet still be bad.
I had coils pass Ohms test but dies when engine is warm to hot.

Bad plug wires, or plugs, can kill system under load and still pass Ohm test.
If you had bad plugs/wires then can kill the coil(s) too.

normal DVM uses 9v battery for Ohms and uses <0.5 amps...
1. Running the parts draws more amps at higher volts. Is about same problem trying to test battery cable to starter w/ DVM Ohms.
2. Coil Primary is 1-2 ohms. Most DVM are useless measuring low values and a shorted Primary can look ok. OE Secondary resistance should be between 5000 and 7000 Ohms but that could have shorted winding and still pass.
3. Insulation Breakdown can happen anywhere. Including internal to the coils. Bad insulation can still Pass standard ohm test.

Scanner only report Volt at ECM and most times doesn't help.
Low volt for any reason can cause low fuel pressure and dieing pump.
Low volt to ignition will cause problem under load.
Just One iffy ground can to this.

"MAP, both read the same under the scaner"
Proper test needs DVM, vac pump and gauge but maybe ok.
Make sure you have right port on TBI for MAP.

"EGR, both org and replacement pass tests"
OE are negative backpressure type. You can't test using vac pump etc. If N type works w/ vac pump then EGR have problems or is dead.

Fuel pressure regulator (was leaking, no change)
700 TBI are known for this.

Back fire thru intake can fry the MAT.
Iffy MAT can lie to ECM and Scan tool and never set any DTC. Many times causes "Fake" DTC 45 at idle in cold weather.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I didnt have the st-125, I made one like it, but the gap is smaller than the st-125 so I am going to get one today.

All tests on componets have been by the service manual. I just tried swaping parts as a sanity check.

Yes it is hard to test the pri windings of the coils, but 6 bad coils? I doubt it.

Today I will try to start,
fuel press
use the st-125
different plug wires
inspect/replace the MAT
check all grounds
harness shake test

I am open to ideas.......

Why did the first engine fail?
My jeep engine went out, swapped in a new engine and the new engine would backfire under load. Clogged cat took out the first engine and would have taken out the second one if I didn't find it.

Make sure the cat isn't clogged.
Check the fuel pressure.
Make sure EGR is not stuck open.

quote Originally posted by sardonyx247: Yes it is hard to test the pri windings of the coils, but 6 bad coils? I doubt it.

Sadly is possible to have several bad coils.
Coil problems are easy when coil have iffy plugs and/or plug wires.
Last year I had old wires went bad and that hosed the coil(s). (1 bad then replace the set because coil made by whoever can change specs and performance.)
Bad coils can hose the ICM too.

note: coil resistance can change depending on maker...
OE resistance above but example Wells says 0.2 - 0.7 Ω primary, 6k - 11k Ω Secondary
http://www.wellsve.com/part...0661240&PART_NO=C849

I've had dead EGR behave iffy ignition too.

[This message has been edited by theogre (edited 02-17-2015).]

quote Originally posted by mike-ohio: Why did the first engine fail? My jeep engine went out, swapped in a new engine and the new engine would backfire under load. Clogged cat took out the first engine and would have taken out the second one if I didn't find it. Make sure the cat isn't clogged. Check the fuel pressure. Make sure EGR is not stuck open.

Well the first engine had a knock, and was drivin with the knock for a year, the engine just wouldn't die, it never did die, just stopped driving it. (Got another Fiero at the time)
The 87 engine in it, came out when I did my quad 4 swap.

This did not happen from the swap etc, I swaped in the engine and sold it to a girl she has been driving this 84 for almost a year.

BTW I forgot to say I allready checked for a clogged cat, I did alot yesterday.


Now today,
First thing, I start the car, runs fine, stab the gas, no pop, nothing.
So I take it for a drive, lug it no backfire, no pop, good. drive about 4 blocks around and decide to try it again, lug it pop, pop, pop, it's back.
so something got hot and failed, I check everything with a laser temp gun, things are only about 97 deg F, so not really hoy at all.
Now having borrowed a st-125 spark tester, and one with even a bigger gap. I try the 125 on one, then decide may as well try the bigger gap, more load.
I get spark jump on all 4 cyl. and this is jumping like a half inch gap, huge load.
Then to check fuel pressure, I hooked up the adapter after the filter. I get about 10 1/2 solid. With the gauge hooked up, stab the gas, pop, no change in fuel pressure.
So good pump and non cloged filter.
Now I have ohm'd the wires before, so I check them again they range from 380-500 ohms, great.
But since I haven't swaped wires to test, I swap wires, I make sure it is still popping to be sure, check again for the pop, and its still there.
Put on different wires, not even new, stab the gas, no pop.
So I take it for a drive, put it in 5th gear at like 5mph to lug the hell out of it, no pop, no backfire. GREAT

Here is what happened, closer look at the wires I pulled off, they don't have the straight boots, they have a 90deg on them, I used them as after doing 2 V6 cars with V8 wiresets I had a set of 4 wires, not a perfect fit with the 90 but ran fine for years. After time the boots stretched out from being crammed on, well when they got hot, just a little bit, they would deform more just enough to leak spark under load.

Thanks all for trying to help, I wanted to finsh up this thread with the results.

[This message has been edited by sardonyx247 (edited 02-17-2015).]

Backfires always seem like a timing issue to me.

Glad you fixed it. I had a bad one once.... it was relatively new.

quote Originally posted by 2.5: Backfires always seem like a timing issue to me.

Yes and most of the time it is, but the 87-88 dukes have DIS, no timing adjustable.

quote Originally posted by 2.5: Backfires always seem like a timing issue to me.

Many times true. Many time not.

Bad coil(s), wires or plug can and often will cause this.
Anything can often pass basic Ω test and still fail at 20,000-40,000 volts.
You need a Megger to test the insulation. Megger test at 500-1000 volts but many times if a coil or wire fails at high volts then can fail at 500-1000 volts too because of carbon tracking.
Many DIS coils fails are cause by secondary circuit problem. Secondary winding will dump spark energy to primary winding when you have bad plugs or wires. Is a well know problem is why many people replace replace the coils when driving w/ bad wires. Wells have a video cover this.

Bad EGR can cause this too. I have a bad aftermarket universal EGR that will backfire where you hit the gas.