I recently bought a 1988 coupe, with the stock 2.5L engine. (not the GT, the formula, or the SE body either) When i give it a decent amount of throttle, the whole thing sputters and backfires. When i lightly press on the throttle, it goes just fine, but is really a drag to drive around as i have to accelerate real slow. Just wondering if this is an issue with the carburetor, spark plugs, timing belt etc.

The most relevant thing i could find online says it is a timing issue, but i have no idea where i would start to deduce what aspect of the timing it would be.

Any help is very much appreciated.

Check the plugs.. if you have bosch platnium... get rid of em.

LMK what you find.

You might have some bad spark plug wires. with the car running try spraying the plug wires with water. If it starts running rough you know that you have to replace the plug wires.

Can not check time control by ecm..Pull plug wires and check each coil compare spark.common problem is coil going bad eratic spark,, a 88 duke will run on one coil,starts running worse and worse..but first check spark from each coil,, make sure you have side electrode spark plug,,some coil pack duke will run ok on fancy plugs,,best wire check is to watch at night maybe spray a lite water spray watch for arcing..my engine was in premo condition and easily ran on 1 coil,(hard to start) coils check for novice?? disconnect one coil(2 wires) spray quick start into TBI,,with warm engine, start engine good coil should run on 2 cylinders never spark coil wire with out a spark plug inside,and grounded, unless you want to replace parts.. old black(sooty,carbon) spark plugs make best check plugs!! save a couple if you ever have for later use,, back fire can be caused by crack ceramic on one plug ,duke runs fairly well on 3 cylinders,, plugs should be gapped at 60 wire gage beat ,,duke runs rough at 45 gap!!align side electrode with center if needed (careful carefull) just inspect close eyeball will tell,,In rough order of predominance standard backfire reasons ,bad hose,vacuum leak,int manifold, top half of tbi loose or bottom,,,bad pickup,bad ground at coil pack(difficult to diagnose),,egr??check bolts and hoses, fuel injector.. sticking valve or lifter(unlikely) clogged fuel filter.bad pump,causing erratic mixture,,bad pump will backfire,day by day if slow deterioration fuel pump going???? worn timing gears,, check spark plugs and compare with chart in haynes manual .good is light brown or grey at top lighter brown at bottom(haynes wrong here) all emission suppose can cause but normally egr or map only cause back fire,, with poor power check engine electrics first.. I have a water proof mouth ,no one can drown it out!!

[This message has been edited by uhlanstan (edited 10-03-2007).]

Mine was a clogged fuel filter once and a clogged cat converter once. Car would not take more than half throttle without bucking and sputtering. The fuel filter is easy and cheap. The cat is a little more involved to replace but a good test is to remove the O2 sensor to see if it gains back some power.

A split hose in the fuel tank above the pump could also cause problems due to low fuel pressure. The split allows enough pressure for low power but asking for more power causes problems.

[This message has been edited by Hudini (edited 10-02-2007).]

Welcome to the best automotive forum on the net! As you can see from the above posts, your problem could be caused by many different things. Don’t shotgun it. Someone here has had the same problem and will supply the solution.

Don’t know how much you know about the 88 but it was the last year the Fiero was made and some things are unique to the model. The brakes and suspension are unlike previous years and since parts were produced for only one year, some are becoming hard to find. To some, the 88 model is the most desirable Fiero because of the improved suspension and brakes.

The 2.5L engine is also a bit different in the 88 because a balance shaft was added and the oil filter was relocated to inside the oil pan.

Both the 87 & 88 have distributorless ignition systems (DIS) so the timing is not adjustable.

HMM....all the electronic troubleshooting has been covered....how about a mechanical issue like a burnt valve?

If it does consistently throughout the RPM range, I would suspect something mechanical.

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If you can't be good, be good at it.

Start with simple things first, check wires,spark, and plugs,try diagnostics that are free before replace parts,, my first post is in rough order of frequency of problem(waiting for flack) a sticking valve or lifter will be more obvious than other problems, the test for this is remove sparkplug wires one at a time to find which cylinder causes least drop in rpm, this isolates problem.use techiques that isolate problem

Any update?? Did you check the plugs?

Have not had the chance to check the plugs, (work and school are piling up) ill be sure to let you know when i do, and thanks to all who have posted advice thus far

Try pour small amount of fuel into throttle body while running and you have fuel in plastic cola bottle with hole in cap(one of my most valuable tools) small hole allows you to control fuel well and avoid grey or no eyebrow,, never try on V 6 but why not??? if an engine runs lean this is easy low tech way to check

Thanks to all who posted here, i had time this weekend to take a look at the engine, and the plugs were in TERRIBLE condition. I found the engine was firing on only 2 cylinders, and when i replaced the plugs and wires, it ran like a dream, runs perfect now, only problem is my "ajar" idiot light is now on permanently, but that is probably a completely unrelated issue.

Again, thanks so much for the speedy response to my questions, I'll definitely be coming back here if i have more issues.

I heard the the 2.5 builds up carbon in the tracts,under the carb, or what ever you call it.
I actualley had this carbon buildup in my first fiero an 84-2.5L.The carbon was so compacted,it lookd allmost like a gasket.If you have a pin hole in your fuel pressure regulater,then unburned fuel travels down the exhaust,and eventualley causes the backfire.

How long has it been doing this, all of a sudden? Was any work done to the engine lately?

it was doing this for a while, but like i said, problem's fixed now, thanks to those who got back to me right away

quote Originally posted by Mongoose736: Thanks to all who posted here, i had time this weekend to take a look at the engine, and the plugs were in TERRIBLE condition. I found the engine was firing on only 2 cylinders, and when i replaced the plugs and wires, it ran like a dream, runs perfect now, only problem is my "ajar" idiot light is now on permanently, but that is probably a completely unrelated issue. Again, thanks so much for the speedy response to my questions, I'll definitely be coming back here if i have more issues.

Kinda figured that was the problem.

Check the trunk wire harness for the ajar problem.