runs great cold rough after 5 min, 1.7 djet Options

[bradtho]

I've got a 1.7 djet engine that I use as my daily driver, so I'm very familiar with how it normally behaves. Recently every time I drive it is great for the first 5 minutes, pulls strong and runs very smoothly. After 5 minutes (I've been timing it and it's pretty consistent) like a switch is flipped, it starts feeling lumpy or fluttery and completely gutless. Very limited throttle response and no torque for getting up a hill. feels like I'm lugging it no matter what the rev's are.

absolutely no bucking involved at any time.

I've done searches for rough running and find plenty with the opposite (rough running cold, smooths out warm) but nothing with this pattern.

Given how well it runs cold, I'm ruling out anything mechanical and assuming it must be one of the DJet sensors that is heat sensitive, like AAR or CHT, but reading pbanders site, I don't see any symptoms like this for any of the sensors.

does anyone have any ideas of things I should be considering to try to help debug this?

[914sgofast2]

Check the AAR valve to see if it is functioning correctly. If the AAR valve is still open once the engine warms up, it will cause a rough idle that jumps all over becasue it is making the fuel mixture excessively lean. I had this problem on my 1/7 Djet. Since good AAR valves are hard to find, I disconnected and removed the AAR valve and plugged the rubber hose ine where it connects to the intake plenum. Car runs much better when hot, but it no longer has a "fast idle" feature when cold, so you have to blip the throttle to keep it gpoing until it warms up. It also has plenty of power once again.

[BeatNavy]

It would be helpful to have more info on your symptoms or what you've checked so far to rule things in or out. Having said that, here are some ideas for you. Others can fold, bend, spindle or mutilate as they see fit:

1. I'm guessing you're probably not running lean if you're not getting bucking or idle hunting and it runs better cold.

2. If anything, I would guess you're running rich. Cold engine likes that, warm engine may bog. Is your idle dropping really low after the car is warm? Are there any other signs (e.g., wet plugs, smell, etc.) that may indicate over-rich? If so, may want to test your MPS to see if it's not holding a good vacuum anymore. Test the CHT and inspect the wiring. Check the fuel pressure.

3. Still could be ignition or timing-related even though it seemingly corresponds to the car warming up. Have you ever cleaned and re-lubed your dizzy? Warmed-up grease may be sticking your advance plates. Check dwell and timing to otherwise rule those out.

4. When was the last time valve clearance was checked / adjusted?

These are some ideas, but we need more data, sil vous plait.

[914_teener]

Check your electrical connections...coil...bat. inside the engine compartment
Particulary the CHT connection.

What could be happening is as the connection warms it expands causing the problem.

[Mikey914]

Check the CHT sensor. Usually the wire is bad so easy to see. You can check the resistance without removing.
Mark

Got them if you need one

http://shop.914rubber.com/Cylinder-Head-Te...m?categoryId=-1

[bradtho]

thanks for replies everyone.

I haven't actually done anything yet besides get frustrated driving my commute. hoping to dig in over the long weekend.

I've theoretically ruled out timing and valves (and other mechanical issues) because it was running great last Friday. Ran good early sunday, then every drive since sunday afternoon has had this issue. that's too instant of a change for something like the valves or timing, right?

I concur with the running rich diagnosis, smells rich on startup. my car has always seemed a bit rich, but it's richer than that lately.

final thing I'm still noodling on is that this coincided with ambient air temps going up in the seattle area. I've joked that we skipped spring this year and went straight from winter to summer. the first day of the warm weather was sunday when this all started. given this tie to ambient air, I'm going to start w/ the AAR when I get the chance to look at it.

if anyone else has some suggestions, bring em on. my experience with DJet is that it's never the first thing you check or what you think it will be!!

[914_teener]

I experienced similar symtoms before my MPS finally gave up the ghost. It started to crack and I noticed too it seemed like it was running rich until it finally broke completely.

If you haven.t already done so check it with a vacuum gauge and make sure it holds vacuum.

[bradtho]

I'm not having much luck...

AAR, functioning well. fully open cold, after running 12v through for a few min, it's fully closed. no pinched hoses.

CHT, 2.3k ohms at ~65 degrees F in my garage. this is right in line w/ what Mr Anders suggests. no nicks in the wire housing.

MPS, I pumped the pressure up to 10 waited a minute and it had lost a little, but was still above 9. well within Mr Anders spec, both circuits show continuity and neither ground to the case.

it continues to run poorly. I haven't had it run well cold in the last several days. I was thinking this was the ambient temp (it was much cooler in the mornings several days ago when it was running well cold).

Any other top-side checks? if nothing else, I'll jack it up and check the valves.

[bradtho]

valves were all fine.

timing was off, I reset (top tip, don't be a dummy and forget the stupid vacuum advance).

Now runs better around 3500, but like crap everywhere else. idle is crazy low. Something is still wrong, so not messing with the idle right now.

My next thoughts:
- maybe BeatNavy is right about the advance plates? explains the timing problems
- trigger points were replaced a year or so ago. maybe the new ones go bad quickly?

any other thoughts of things I should check?

[BeatNavy]

Crazy low idle could be confirmation that you're running way too rich (esp. assuming you got the timing right - did you check dwell first?). Really, the primary causes I can think of for that include:

1. Failing MPS (which you checked - scratch that off).

2. Faulty CHT / CHT circuit - you checked at ambient. You should probably check with engine warm as well and measure results against Anders' numbers.

3. Fuel pressure too high.

4. Leaky or stuck injectors or CSV. You can disconnect the line to CSV and at least eliminate that as a possibility.

It's probably also worth checking that the T1 sensor (Ambient Air Temp) is connected and working. It's a small overall impact but combined with something else it could aggravate a rich condition.

This seemed to happen rather suddenly, so I wouldn't necessarily think something like fuel pressure, but it's easy enough to check. I'd definitely verify CHT values when warm, verify dwell AND timing, and yes, clean and relube dizzy.