Runs rough or stalls when started warm, otherwise OK

[benwood]

My 1991 Mazda Navajo LX has 127k miles on it. I discovered a problem recently, and have been trying to narrow it down. I did some routine maintenance that could have been a cause, but apparently was not.

Pattern: when engine is cold, starts and runs OK. When started warm to hot, it will run extremely rough (or stall) for 10-15 seconds, then seems to be normal for as long as car is run.

I've put in new plugs, gapped to 0.54 (old plugs were only 15k but were also 8 years old, and gaps had opened up some). Car runs smoother now except for warm/hot start.

I also replaced ignition cables (never done before) and air filter. And I had the fuel filter replaced at my oil place last week.

It definitely runs smoother than before, once it is going, but the warm/hot start problem remains.

My daughter just brought it home, so I went out to listen for any sounds like stuck fuel pump (I had the relay fail about 15 years ago, keeping the fuel pump on all the time), but couldn't hear anything. When I turned on the ignition, but before turning over the motor, I can hear systems powering up -- seems different than before, but not sure. I did have a new power steering pump put in about two months ago, due to it leaking. But the first time the start issue occurred was quite a few weeks later (likely a red herring, but I'll throw it out there).

It almost sounds like it's running on 5 cylinders for a while, which is why I zeroed in on the plugs/wires.

Seems like all that is left as a possibility is ignition (but no problem with cold engine, so far anyway), or fuel injector (why normal on cold start, but not hot, and it runs fine once I get going), or some issue with the fuel pump perhaps?

Anyway, any thought would be appreciated.

 

[MrShorty]

Hard warm starts like that are a classic symptom of a fuel pressure regulator diaphragm leaking. There's a vacuum line attached to the FPR that is part of regulating fuel pressure. When the diaphragm leaks, it leaks excess fuel into the intake through this fuel line -- effectively flooding the engine. When you try to start the engine while all that excess fuel is there, if it starts it will run rough until the excess fuel is cleared out of the intake. If you wait long enough, the excess fuel dissipates on its own, so cold starts are just fine. Diagnosis is simple. Locate the FPR and the vacuum line coming off of it. Run the engine for a few minutes, shut down, and pull the vacuum line off of the FPR. If there is any gasoline in the vacuum line, the FPR needs to be replaced.

 

[kmack]

While it could be a fuel pressure regulator, I think it's more of a bad ECT sensor.
Check this thread: https://www.explorerforum.com/forum...ne-shudders-and-runs-rough-at-startup.458560/

Post #11 gives an explanation as to why.

 

[kmack]

Mine does this, but it hasn't made me mad enough yet to fix it. I have ruled out the fuel issue.

 

[RustyMacintosh]

DITTO on the replies...I have a 94 XLT with 217k, new heads--it would drink a bit of antifreeze prior to swapping in new heads. Further, after the top end was replaced, it formed a strange misfire

COLD--smooth. But if you drive it for lets say 30 minutes. Park it. Run into the store, come back there is a misfire. I would hold the RMP's at 1500 for 15 or so seconds, then all of a sudden---2000 revs and smooth.,

We thought fuel injector leaking...nope fuel regulator

 

[benwood]

Thanks so much for these two important leads. They both sound plausible. I won't have time to try a cure until I get back from a trip to Arizona next week. It was a hallelujah moment when I realized there was a pattern to it, and that it is recoverable (but embarassing... heh heh). My daughter understands the issue and mostly is only doing cold starts, and she's comfortable for now. I will be thrilled to take care of this myself. I've done quite a few things to this vehicle myself, including water pump and new fan, tensioner, alternator, serpentine belt, batteries (of course), tune ups. Managed to get that nasty 6th plug out from the top last week, had to do that from the bottom before, blind. Just kept at it this time. Almost 28 and a half years old... I had no idea I'd keep it around so long, but as a backup vehicle for the past 25 years, so only 127k miles.

Anyway, I really appreciate the input and when I get to this in later April I will report back. Hopefully I'll figure out where the necessary hose/ECT sensor is for this.

 

[benwood]

Well, I am reporting back! I decided to replace the ECT sensor, since it looked like a much simpler job. I just bought the part locally for $25, and surprisingly (for me working on my car) it only took an hour from start to finish. I took my 1991 Mazda Navajo LX for a test drive, letting the engine get hot, and then first waiting 10 minutes to restart (OK!), then after another ten minutes of driving, I waited five minute (OK!), and then I drove home about 15 minutes, and tried after a few seconds (OK!). Not definitive, but so far so good. I thought I'd share some photos since photos always help me.

First, the old part & box that contained the new sensor. Second shot is of the new sensor going in under the throttle body. Last is fully screwed in with the electrical connector reattached.

And thanks for the reminders everywhere to disconnect the battery before I did the work. I'll report back later if this turns out not to have fixed the issue. If that is the case, it will have been narrowed down to the fuel pressure regulator. I probably would punt that for the shop.

And geez... this car is getting old. I also had to hot glue on the driver door pull, which keeps detaching, and run a tight circle of electrical tape around the driver side door catch (cheap fix, need to do this every few months).

...

 

[benwood]

couldn't get rid of that duplicate photo...