Mid-RPM stumble gone

[mejulihn]

This topic comes up from time to time. The usual suspect is a bad throttle position switch. But having replaced mine with no improvement, I have been frustrated in diagnosing the cause of a mid-RPM stumble. Finally took the car to my pro 914 wrench for some other work and asked him to also check the timing. When he checked, the factory timing marks were no longer visible on the fan. He removed the fan and used the template I downloaded from Pelican Parts (thanks, PP!) to add new marks and reset the timing. Turns out the spark had been very retarded (no remarks about the owner please). Just reaffims what DD and others always suggest, check the basics first!

[rdvnac]

I love happy endings, but I especially love happy endings involving simple solutions that I may be able to use myself someday when facing a similar problem.

Thanks for sharing this, mejulihn.

[pbanders]

It took me a long time to learn it, but for general problems, you go through it this way:

• Basic mechanical condition - adjust valves, vacuum gauge test, check compression
• Ignition - dizzy mech/vac advance & retard working; plugs, wires, points, coil, condenser, cap, rotor, all new or in good condition; plugs gapped to spec; timing and dwell to spec
• Injection - All hoses in place, no air leaks anywhere; all connectors and boots in place; no shorts; fuel pressure to spec; sensor resistances ok; MPS holds vacuum; etc.....

Just doing that stuff will usually get rid of most problems. For more detailed FI analysis, go through Kjell Nelin's procedure here on the PP web site, and check your components using my D-Jet parts web page.

[mejulihn]

While he was at it, I also requested the shop check the head temp sensor (TSII). Yep -- needs replacement as well. It is on order and will be installed next week. I see that is on Brad's list as well!

[pbanders]

Just interested - what did he find that was wrong with the sensor? Open? Shorted? No resistance variation with temperature? Wrong resistance values? Did the car run with the sensor in this condition? thanks....

[mejulihn]

He told me the resistance was over 4K ohms cold (I think you previously posted it should be 2K). I am not sure when warm but it runs fine after it is warmed up. I have recently had a severe bucking for the first few blocks when driving it cold. I am hoping this is related. I will let you know more when I have it installed.

[pbanders]

Hmm. First, if it was cold enough, 4K ohms may be ok. The 2K ohms figure is for room temperature, 70 deg. F or so. If it was as cold as in the 40's, 4K is probably right. Bucking after starting is usually an indication of the car being too lean. Too rich and you get bogging - push on the gas and no power, stinky exhaust, easy to tell/smell. Since the higher the resistance of the head temp sensor, the richer the mixture, bucking doesn't fit what I would expect.

Did you check the fuel pressure? Too low = too lean. How about the injectors? Clogged = too lean. Condition of your plugs? Fouled = crappy ignition = bucking. Timing and dwell? Can your mechanic put it on an A/F meter and tell you if it's lean or rich when at part-load when cold?