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...

 

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.

 

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).

 

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.

 

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.

 

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.

 

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?

 

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

 

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?

 

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