Changed Computer, now backfires
 

My engine control computer (PCM, ECM, ECU whatever you call it) went bad on me. The symptom was the truck would die as if you turned the key off. I checked all the solder splices in the wire loom and repaired three connections of 4 wires each (Blue, Red/White and a group with 2 black and two gray or white wires). Problem continued. Finally figured out that whacking the computer black box under hood with hand would cause the fuel rail to charge and check engine light to return (and thus restart truck). I figured computer had intermittent short so I bought a replacement at NAPA for $189 exchange. The number (56027141) of my old computer was matched to a new computer.

Since replacing the engine computer the truck now runs rough and backfires. It idles fine in driveway and rev'ing it in park/neutral seems fine but when you drive down the road it runs rough at approx 2000 RPM and backfires too. Am I supposed to do something to the computer to match it to the truck? Could I have gotten a bad computer? Do I just need to run it a while to give the computer time to tune itself to match the truck? Any ideas?

Thanks in advance for any help.

RE: Changed Computer, now backfires
 

I should add that no codes are set (except 12 but I disconnected battery when I changed computer). I've held various RPM's in drive way in park (up to about 3000) and it runs smooth. Its only under load while accellerating that it does this. If you romp on it it accellerates fine but pulling away from a stop sign like a regular person (ie not floored but not grandma style) it runs really rough between maybe 1500 and 2400 RPM's, after each shift when rpm's drop down near 1000 - 1300 its fine again til you get back near 1600 or so RPMs. I've not driven it much since swapping the computer. Couple short trips ie round block to test repair, ran it a while in driveway in park, wife went to store last night, etc. Maybe 5 miles driving since replacing computer. Called NAPA (where i got remfg computer) and they didn't know if anything needed done to computer. I read something about matching the VIN inside the computer via an EPROM flash? Is that for gen 2's or also gen 1s? How would computer know the VIN of the truck? Is there another module that tell it the VIN?

I need this truck for work tomorrow and its not going to be fun driving on the freeway if it backfires constantly at 2000 RPM's which is about what it takes for 60 MPH freeway driving... I'm worried I'm going to damage the catalitic converter or muffler too... Kind of afraid to drive it. any help?

RE: Changed Computer, now backfires
 

Spark plugs gapped properly? With the Powertrain Control Module number you gave (#56027141) it appears you have a 5.2L. Make sure the plugs are gapped @ 0.035"

Also make sure the spark plug wires are in good condition. I remember when a spark plug wire on my grandmas Dodge Shadow went bad, only under acceleration of certain degree (not giving it too much gas, enough to accelerate, but not too much) it would do the same thing. Grandpa tracked it down to a spark plug wire.

Also make sure the distributor cap and rotor are in good shape.

Also check all the grounds you can find (mainly the ones mounted on the firewall and near the PCM).
Post back with results, questions, etc.

RE: Changed Computer, now backfires
 

also not shure if its true or no but at 1 time i was toled that alot of the dakotas got a thing w/ the ecm or whatnot that with some canges ie heads headers exc... the comp gets used to running the way it is and when u make a change to something like that or let it sit for winter and whatnot u should let the truke run i belev at idalfor about 45 min or so to let ecm readjust but like i said im not to shure about that whats your thoughts on that dodgerules86

RE: Changed Computer, now backfires
 

I'm not sure about that long.
I know after you "reset" the computer (disconnect the power from the computer - or disconnect the battery - for about 5 minutes or so) it goes through a re-learning procedure. I've heard that after you reset the computer, to drive around for a few minutes so the computer learns again.
A few months ago, my grandpa had to install a new PCM in his van (it was having problems) and he noticed the differance as soon as he started the van with the new PCM.
Just check the grounds, make sure connections are good, tight, corrosion free, and post back.

RE: Changed Computer, now backfires
 

Well basically I was looking for a response indicating some special procedure you must follow after replacing the computer to sink it to the new vehicle. You see the truck was running fine on Saturday less than an hour before the computer swap under the old computer. The old computer had a problem where it would lose power to itself which rendered the truck useless, crank all you want it aint gunna start because the relay box and coil are getting no power. No fuel and no spark. The intermittent loss of power to the computer is gone with the new computer but the new computer makes the truck backfire, it did not do this prior to the switch so I'm 99.9% positive that its the remanufactured computer thats screwed up and not some coincidental failure on another truck part.

I talked to the local dodge dealer today and confirmed that there is nothing special to do after replacing the engine controller other than ensuring the computers part number matches the one you are replacing (it does). The dealer also indicated that 1 out of every 2 or 3 reman computers are defective. Wish I woulda gone to the junk yard or to the local MOPAR store instead of NAPA. The local MOPAR place had two of these computers in stock... DOH! Anyway, NAPA has another one in route from portland to spokane and it will be here wed. Until then I just have to avoid highway travel as it runs fine under about 1700 RPMs. Over that and its constant backfire.

Question... Anyone know of a sensor I could unhook that might make it run in that hobble home mode to see if it runs better ie no backfires at 1700-2400 rpms or so.... ??? Looking for a temp solution til the new computer reaches town. I have 100 miles of travel approx before I get new part. And some of that requires highway driving (to/from work) as far as I can tell... I can find now alternate route that keeps me off the highway. It doesn't backfire if you grandma it or if you romp on it while accelerating, only if you take off normally so I've been romping it... gettting to speed (30 or 35) then grandma driving to keep speed. Today I was able to do highway speed (60mph) by turning off the overdrive but that means high RPM's (3400 or so) which I dont like. So If anyone has an idea of how to trick this faulty computer into working better I'm all ears...

RE: Changed Computer, now backfires
 

No clue. Sorry. But keep us posted as to if the new computer helps.

RE: Changed Computer, now backfires
 

New computer arrived early, picked it up and installed it in the NAPA parking lot today. Guess what... Truck still backfires...

I still wonder if its something to do with the remanufactured engine controller (ECU or whatever you want to call it) but I figure I'll get laughed out of the store if I go back to NAPA demanding yet another unit so I better do some trouble shooting first. So... Once again... What are the possible causes of a backfire in tail pipe (running rich) condition that does NOT set trouble codes?

I just got home from picking up the new computer so I've yet to tear into things but after posting this I'll be headed out to remove spark plugs and see if its running rich on all cylinders or just select one or few. I'll post back with results, even if I find the problem.

I might also note that a little history to this can be found on the dodge dakota forums, here is a link:

http://www.dodgedakotas.com/boards/v8/7278.html

EDIT:

I should also note that after installing the first new PCM I let the truck idle for a good hour and put over 100 miles on it before replacing with second PCM, no change in backfire. Second replacement PCM was installed today, didn't idle it for any length of time but I have put a good 35 miles or more on since installing it. And again there is backfire condition.

So far I've pulled 3 spark plugs and they look good, whitish gray color with a tinge of brown where the spark goes (underside of the metal tab that is gapped). I'm not gunna pull the rest because I can prevent the backfire depending on how I drive and my last time driving I was preventing the backfire. I guess I'd need to take a drive and cause it to backfire on purpose then pull the plugs to see which cylinders are running rich. I dont have time for that now as I have to pick up my wife in less than an hour and the manifold is real close (and real hot) to the spark plugs.

RE: Changed Computer, now backfires
 

So far I have swapped out the coil (I replaced it last summer but that didn't fix a previous prob so I assumed the other one had still been good), changed spark plugs and done limited testing of plug wires. I had a new set of wires for a chevy V6 so I was able to use 5 of the wires to selectively test the 8 connections. Problem is the chevy set has no coil to cap wire since the chevy has a coil pack deal and no distributor. All old plugs looked good, white/gray with brown tinge. I did put in some injector cleaner yesterday and so far have driven about 70 miles and I should admit that the last 10 - 15 miles it was running better. still a bit of hiccup at 2000 rpm but not the wild horse jumpin round like before. I could actually sustain speed instead of having to constantly throttle jockey between 55 and 65 to prevent the backfire. Hopefully the injector cleaner will do the trick. Thinkin I'll fill up again tonight and put in more cleaner.

I'm sure its not the O2 since the backfire occurs at all times (cold or warm engine). I heard that unhooking the EGR might make it run better indicating a sticking EGR valve, is that true? What else might cause backfire? I know I should check the wire from coil to distributor too but I dont think the wires are bad.

I read something about open vs closed loop control via the computer. How do you check which mode its running in? How do you force it to run in the other mode? I think I read that you unhook the coolant temperature sensor, is that true? If that makes it run better whats it mean? bad coolant sensor or something else?

RE: Changed Computer, now backfires
 

One of the main sensors the PCM uses is the Engine Coolant Temperature Sensor. This is located near the generator (its a 2 wire connector, do not confuse it with the 1 wire connector on the engines right of the radiator hose, that is what feeds your coolant gauge). As coolant temperature increases, the resistance of the sensor decreases.
Once the PCM determines the engine has reached "Operating Temperature" it goes into Closed Loop.
You can test the engine coolant temperature sensor, and the intake air temperature sensor (located on the front of the engines right intake manifold) by checking their resistance, via the chart provided.
The main thing that changes in Closed Loop mode is the Oxygen Sensors are monitored and used to maintain a 14.7:1 air:fuel ratio. (Closed loop modes are: warmed-up idle, cruise mode; open loop modes are startup [engine cranking], warm-up, acceleration, deceleration, and wide-open throttle.)

[IMG]local://upfiles/948/B3189619F01B436381E620E778985543.gif[/IMG]

RE: Changed Computer, now backfires
 

Been busy and haven't been workin on this issue lately. Bout to move and been painting the new house top to bottom before we move in.

Anyway, I just checked the resistence of the coolant temp sensor at work. truck been parked for about 5 hours after a 40 mile drive. Its approx 75 degrees here today. ohms read at about 5250 indicates approx 104 degrees. Started truck ran it about 5 mins then took another reading, this time 4000 ohms indicating approx 120 degrees. I'm guessing that is acceptable. I'll drive home with sensor unplugged tonight to see if it makes any difference. Maybe it will run better in open loop, who knows but worth a try...

Question: At what temperature does the PCM consider to be "operating temperature" where O2 sensors are in play? Its been darn hot here lately (cooler today) with highs near or slightly over 100. The truck doesn't backfire til I'm 2-3 blocks down the road so maybe thats all the time it takes to warm up? I dont idle long before pulling out.

Well hopefully open loop mode will make it run better so I have some better idea of what parts might be wrong. What all sensors are ignored in open loop vs closed? Still wondering if I have yet another defective reman PCM... :(

RE: Changed Computer, now backfires
 

I would try and find out what side of the motor its coming from. might make it easyer to find. have some on rev it for you or do it your self with the trottle body. they only thing i can think of is mabe the fuel pump is starting to go. if your sure the back fire is coming from the tail pipe then you have me. but if you hear it from under the hood it might be the fuel pump. I just thinking that at one piont the fuel pump can't keep up with the motor. only other thing is check the timing with a timing light. I may be going off in the wrong direction but you never know intel you find out.

RE: Changed Computer, now backfires
 

What should I do... run along side the truck while someone drives it on the freeway? It doesn't happen at idle or rev'ing it in neutral. Only under load, particalarly at freeway speed (50-70mph).

RE: Changed Computer, now backfires
 

Sorr forgot about that. the only thing i can think of is some where in the intake or exhaust, or maybe injectors. If i remember right you said its not running rich. maybe its running a little lean. Fromwhat i know a backfire is caused from the fuel system, intake or valves. Another words timeing might be off. other than that man i am just as lost with it as you are.

RE: Changed Computer, now backfires
 

Sorr forgot about that. the only thing i can think of is some where in the intake or exhaust, or maybe injectors. If i remember right you said its not running rich. maybe its running a little lean. Fromwhat i know a backfire is caused from the fuel system, intake or valves. Another words timeing might be off. other than that man i am just as lost with it as you are.

RE: Changed Computer, now backfires
 

So now that I know the difference between the air intake temp sensor and coolant temp sensor.... With the coolant sensor unplugged my backfire problem has gone away. Nothing that concerns me as I expected there may be weird behaviour with the coolant sensor unplugged but now the idle bounces a bit (most noticable in neutral or park) and shifting from rev to forward or forward to reverse often causes truck to die. Anyway, I found a chart that shows what sensors are used in the various modes of open and closed loop operation so I'll review that to see what sensors are suspect (in case its not the O2 sensor) before I buy parts.

Here is link to the chart I found, not totally sure what year of dak's its for but I'm sure its similar at least. http://www.v8performance.com/tipsand...htm#openclosed

Also, if anyone has any ideas based on this new info I'm all ears. I'd rather troubleshoot a problem and buy the correct part first time than just throw money at it till I stumble on the solution... Call me cheap but yeah LOL.

RE: Changed Computer, now backfires
 

After reviewing the chart for open loop warmup vs closed loop cruise modes I think the only sensor difference is the O2 isn't in play in open loop. Would it be worth while to reconnect coolant sensor and unhook O2 as a last test or is it now safe to assume O2 is the culprit?

RE: Changed Computer, now backfires
 

could it be something simple like a summertime gas mix, i know my 89 wont run right on high octanes in the summer, it needs the cheaper gas in the summer

Oxygen Sensor?
 

Ran fine last summer no matter what gas I used and temps got to similar ranges. Since the backfire started I have tried premium gas but normally run the cheap stuff. Wish it were that simple. I should mention I have a hole in my muffler (had it since last winter) that has gradually gotten bigger. I have worried that air is leakin back from there and messin up the readings but it seems unlikely since O2 is in front of catalitic converter and muffler is behind that and hole is on backside of muffler. Seems it would be more likely that the O2 would see weird readings at idle more likely than at cruise as pressure in the backward direction is less at idle.

My best guess is that the intermittent computer (PCM) took the O2 sensor out along with it since the truck ran fine one day and next day (after replacing computer) it started backfiring at cruising speed (1600 - 2400 RPMs approx)

Also, I've heard horror stories from some people about O2 sensors failing often after replacing the first one or having to get from dealer instead of aftermarket. Not sure if that applies to these trucks, anyone have any experience with aftermarket O2's on these gen1 daks? Anything to avoid or be aware of?

RE: Oxygen Sensor?
 

if its running crappy at highway speeds maybe ur cat is clogged up,i kno that will cause it

RE: Oxygen Sensor?
 

Considering unplugging the coolant temp sensor eliminates the backfire miss stumble condition at cruise speeds I doubt the cat is the problem. Nearly certain its the O2 sensor, plan to get a new one installed this weekend.

RE: Oxygen Sensor?
 

OK so I let this repair slide for a while... Was still driving the truck some but not alot as I was using a diff vehicle for work. So I got a new O2 (bosch OE style from autozone) and installed it yesterday. Didn't drive the truck yesterday as I was possitive this would fix the problem. Decided to drive truck to work today and sure nough, damn thing is STILL backfiring. It was pretty cold this morning so took a while for warm enough to use the o2 and it was running fine then but once warm and o2 came into play bang bang bang... It does it at about 1600 to 2400 rpm's in any gear and at any speed at approx those RPM's while maintaining speed or at VERY SLIGHT accell or decell... A previous test of unplugging the coolant temp sensor (history above) resulted in the backfire problem going away so I KNOW it has something to do with open vs closed loop and highly probable is the o2 sensor. Guess now I suspect wiring of o2 circuit, how to test? Any other ideas that might cause this?

One other thing, I have a pretty bad hole in my muffler. Could this mess up the o2 reading? Seems the o2 is a LONG way from the muffler. o2 is at the Y pipe ahead of the cat converter and muffler is way at back of truck bed. I've been holding off on getting a new muffler until the backfire is resolved as I dont wanna blow out a brand new muffler with this problem but maybe the muffler IS the problem???

Any other ideas?

RE: Oxygen Sensor?
 

had the very same thing happen to me check your positive battery cable for breaks etc and also the small wire from the starter it sounds like you have a short

RE: Oxygen Sensor?
 

check the timing..sounds like a timing problem the computer controls the timing.

RE: Oxygen Sensor?
 

Have you found out what your problem is yet? I have the same problem and cant find it

RE: Oxygen Sensor?
 

Unfortunately no. I have not looked into the positive battery cable as suggested above and should do that. I'm also tempted to buy a new coolant temp sensor as with that unplugged the backfire goes away. I had done some simple testing of the temp sensor before but who knows... But that part is only like $12. I also need to investigate the wiring to the O2 sensor. As mentioned before, replacing the O2 sensor did not fix the problem.

RE: Oxygen Sensor?
 

To me it sounds like maybe the fuel sync is off a little bit. That could explain the backfiring, You might want to take it to a dealership and have them set the fuel sync. Have them check the fuel sync and reset it as necessary.

RE: Oxygen Sensor?
 

Have you checked your MAP sensor yet. I had a truck once that did this and I eventually tracked it down to a MAP sensor going bad.

RE: Oxygen Sensor?
 

O2 sensor or fuel pump my 92' Dakota 318 backfired with the same symptoms u r having sounds like a fuel problem to me not the computer

RE: Oxygen Sensor?
 

depends on if you have 2 o2 sensors