97 HB stalling on idle, after reaching temp

[Bannor]

Quick background: 236k, owned the truck for last 10yrs/100k miles, mostly stock parts, just changed timing chain myself a couple of years ago.

The symptoms that the truck is having is that once it reaches a stable operating temperature, the engine will idle roughly and often stall. It doesn't surge on normal idle, only when it has been driven for a while.

I checked the OBD code at a local auto parts store, and it came back P0400 - EGR. Looked around the forums and internet. I took off the air filter and holder, and visually inspected components, checked vacuum lines(no hissing heard), electrical connections, etc. I also took apart the main bundle of wires that runs along the top of the engine, with the intention of replacing the poor grounding clamp that grounds several of these intake components. From firewall to components, I found no such clamp.

I ordered a bluetooth OBD reader to use with my android phone, and used it today.

Checked for fault codes, and it returned P0400. It also, however, had a pending (unverified) P0110 - Intake Air Temperature Sensor. I cleared the codes, and drove the truck around for about half an hour before it started acting up again. Right when it suddenly stalled, the check engine light came on again, but curiously the only fault code it reported was P0110.

On starting it back up it died again instantly, and when I kept it running by pushing the gas pedal down a good bit, it sounded like it was missing. Lots of engine shutter, smell of unburnt gas in exhaust, etc.

Suggestions?

Addendum: Waited 5 or 10 minutes, started right back up and idles smoothly.

[Bannor]

Addendum 2: I forgot to reconnect the IAT to the harness, derp. Thus the new code. Reconnected and cleared.

After driving it around again, I managed to get some real time information at the moment that the engine began having difficulties:
MAF: Jumped from fairly low numbers to 206.7g/s, and stayed there.
AIT: Was at 50C, not sure where it was before.
Coolant: had reached 78C.
Timing Advance: which was at 11-degrees, suddenly dropped to 4-degrees, and then oscillated between 4, 6, and 8.
Engine Load: jumped to 100% and stayed there.

[Q451990]

If I had to guess I'd say that the MAF is the issue... My neighbor, who owns my old 97 HB says that he has to take his apart and clean it from time to time to deal with a rough idle issue. If I remember correctly it's located in a plate that sandwiches between the throttle body and intake? If it's not to hard to remove, I'd try pulling it and spraying it with some brake parts cleaner...

Heath

[Bannor]

Alright, Ill give that a go. I suppose I probably ought to clean the whole intake manifold, it does have 235k miles on it. Ill give it a try after the weekend and let you know how it goes.

[justadodgeman]

I don't know why anyone hasn't said this but since you did throw an egr code then that would be the first thing I checked for a messed up idle because it would be a vacuum leak if it wasn't closing all the way when you came to a stop because it should only open above 2,000 rpms I think and it closes at idle unless its clogged. id say it would fix your problem if you replaced it or cleaned it and the air temp sensor I would also clean you maf sensor and air temp sensor though both with maf sensor cleaner not brake parts cleaner

[justadodgeman]

btw mine was doing close to the same thing. at operating temps every time I would press the clutch in it would almost die then go back to proper idle after dropping down to 300 rpms a couple of times haha it turned out to be a fuel injector that was going out then finally after a few days it went completely out and I replaced it and problem was fixed. ohms should be between 11-14 I think mine was at 56 ohms when it finally stopped pumping fuel

[Dattebayo]

Engine running rough and dieing when up to temp are classic symptoms of a bad fuel regulator as well, just FYI. But usually as it gets hotter, it will eventually not start at all. Check for fouled plugs and get a pressure gauge.

[Desert Rat]

EGRs themselves can clog up with carbon and cause stalling. Pretty easy to remove the EGR valve and look inside. If you have massive carbon buildup inside, you're probably on to something. You can get spray chemicals at the parts store that'll break that stuff up and you can probably save the part provided the diaphram inside isn't damaged.

[Bannor]

Just a quick update, I was preparing to tear down the intake and clean it piece by piece, when I found a small, 4-inch piece of vacuum tubing that had rotted out. It connected from the EGR to the pancake thing right above the EGR.

On reconnecting it (and running a bottle of Catalyst Cleaner through the system), it ran much better.

However, after several days of daily use, the behavior has intermittently returned. Sometimes it runs great, other times it chokes and dies. Previously the behavior was constant. So I'm guessing that stuck valve suggestion above is still valid, so I still need to tear down the intake and clean.

Could also be the MAF, since the OBD is reporting that code now.

Thanks for all the suggestions and info everyone, that will be useful stuff on this adventure. ^_^

[cheesecrakr]

My 97 hb is doing the same thing. It started out running rough after it has been running a long time. I got an EGR code, so I pulled it and it was carboned shut, but that shouldn't cause the problem. I replaced it, but it still ran badly. I replaced the MAF with an aftermarket one and it ran pretty good for a while. About a week later, it started stalling when it was cold at startup. I can make it run by holding the gas down, and once it warms up a little it will keep running, but poorly. I noticed it seems to run better with the vacuum line to the regulator unplugged, or with a finger laid over the air horn. Movement of the air filter housing makes it act crazy, stumble, stall, and all sorts of erratic behavior. The check engine light is not coming on. It's an automatic transmission. The plugs, wires, cap, and rotor are new. Fuel filter is new.