1990 Volvo 740 GLE, 276k miles. Hi all, question here for someone who knows more about this than I do (which is basically anyone). I recently flushed out my radiator to fix a problem with it being airlocked from when I had to fix a problem with the radiator a few days back, and to get out the "additive" which I had added to plug a hole in the radiator (will never use these additives again as they just gunk things up and don't work). After flushing the radiator with clean water, I put in the flush solution, filled the rest up with water, left the cap on the overflow tank off, and started the car to allow it to cycle through. The car started incredibly roughly and has continued to run that way. I did not mess with spark plugs or get any water in that area so I am not sure what the problem is? I ran it around the block for a bit and it still runs rough when it's near an idle, and when you step on the gas there is a delay before it accelerates. I was thinking maybe air in the radiator somewhere? and if so, how do you bleed the radiator?

 

Unrelated problem. Rough running not related to coolant circuit. Look for a disconnected vacuum line that you accidentally knocked off while doing your expert work under the hood.

 

thanks for the suggestion but thats not the issue. I was careful to not mess with any of the vacuum lines, and everything has been triple checked to make sure it's all back in proper place. I have replaced the entire radiator before and did not have any issues.

 

That's impressive certainty for someone who thinks air in the radiator is causing his engine to run rough. So everything has been triple checked and all is A-OK. Glad to see you're on top of the situation.

 

Well I do appreciate you being a smart *** for sure. I have done enough looking into the problem to find that other people have had other identical issues (and it was not as simple as a vacuum) after flushing the radiator. In some cases it was caused by air bubbles in the radiator or hoses and bleeding the system helped, sometimes it was another issue. I am simply trying to see if anyone has experienced similar issues and what they did, whether there was a simple way to bleed the air out of the radiator (besides opening the cap and letting it idle for a while), if there is something else (besides vacuum issues) that I can look into? Like I said, I replaced the radiator previously with no issues, filled the car with new coolant, left the cap off to run it through the system, etc.

 

Vacuum lines or water where is doesn't belong would be my guess.

 

Just out of curiousity, when you performed all of this work, did you disconnect the battery? If you did, it will reset the computer which will have to re-learn the settings where it was running prior to the reset.

Also, what do you mean by running rough? Is that running on 3 cylinders or just missing here and there?

If it were me, I would troubleshoot this as if it were a separate problem. Start with the basics - check for vacum leaks, leaks in the large accordian intake tube, check the throttle body for gunking up which will mess up the idle, check the idle air ciruit. If you have a Bently or Haynes manual, it should have procedures for each of these.

 

Act1292 - Thanks you were dead on.
"Just out of curiousity, when you performed all of this work, did you disconnect the battery? If you did, it will reset the computer which will have to re-learn the settings where it was running prior to the reset."

I didn't see this posting before I figured that out, but you were right. It was the computer being reset when I took a lead off of it while working. The thought had crossed my mind that it would effect it some, but I didn't think it would make the car act quite as out of rhythm as it was. It's slowly relearning how to run properly.
Thanks!

 

Your welcome. Glad I was able to help you out.

 

How long does this "learning" take? Is it two or three operating cycles, or twenty or so? I think I read somewhere that a cycle involves any start-up-to-engine-off where RPM goes over 3000. However, I've never read how many of them are needed for this "learning" to happen.

 

I'm not up on the details of the programming of the ECU but I read somewhere here that driving somewhere betwee 20-30 miles for it to adjust.

 

I also would like to know how many cycles it takes. I would say I am on about the 12th cycle described by you above. It seems to get slightly better with each cycle, but it definitely has a good bit of learning left to do.

 

I dunno, I've never heard of a forgetful LH ECU being *that* far off. the range of 'learning' is pretty limited, and mostly relates to anticipating how much fuel will produce what O2 sensor change at what RPM and airflow range and has more to do with getting that last 1mpg out of things and 1% of clean air.

this isn't nearly as complex a system as later Motronics stuff.

if your engine is running /rough/, I'd be curious if 1 cylinder isn't firing properly. after putting 5 or 10 miles on it, I'd pull the plugs and compare all 4, see if one is noticably different color than the rest, and go from there. I might do a compression test, is one cylinder significantly different than the others ?

I might just do an 'italian tuneup', leave it in 2nd, and wind out out with full throttle as far as it will go, then let it shift to 3rd (D with OD disabled), and do the same. keep it at high RPMs for 5 minutes on the freeway (yes, you CAN go 60-65mpg in 2nd, and don't worry that it sounds like its working hard, thats the whole idea). this will burn off any carbon build-up on the pistons, heads, valves. If you really want to go all the way, put a can of Seafoam in the tank, HALF fill it with gas, then drive that whole half tank out keeping the RPMs as high as practical, like freeway in 2nd/3rd. when you've run it as close to empty as you dare, refill with fresh gas, drive that tank out normally, then change the oil and filter (that fuel cleaner stuff tends to leave your oil pretty much wasted).