Hi Guys, I'm trying to hunt down a very intermittent issue on my 85QV. It starts, idles and runs great. But one in every 100 times or so, when I approach a stop, the the engine rpm just drops to zero. Sometimes I can prevent it from dying by giving it a little gas, and then it idles fine. If it does die, it starts right up again. I've replaced all plugs, wires, caps, rotors, injectors and extenders as part of general maintenance and the performance definitely improved, but that occasional drop to zero remains. I've run additional grouding wires to the digiplexes and replaced two of the 3 crank sensors (but I'll bite the bullet and struggle with the difficult one (10 o'clock position) this week. Anyway, looking at the digiplexes, I see that I've got 2 X MED 801 A. Online, it seems most 85 QV owners have the 805 A. Do I have the wrong boxes? BTW, the vacuum lines to the digis look good. Any thoughts or advice welcome!
I don’t think the issue you are having is from the digiplex modules. There are two versions of digiplex, emission cars and euro cars. Which car do you have?
Thanks! I Agree. Doesn't seem like it woudl be digiplex, but I'm investigating all options. And seeing that most QVs have 805As got me wondering. Also, the fact that the dying happens right after a big rpm drop led me to at least consider a vacuum issue. My car is a US QV...
Check the one way valve in the brake booster vacuum line (next to the plenum) see if its stuck our obstructed.
Thanks Guys. Could you point me to the location of the brake booster vacuum line / valve? It isn't immediately obvious looking in the engine bay and I can't seem to find it in the shop manual. The intermittent-edness defintely behaves like something is stuck when I'm driving and drop the rpms. FYI, this can happen even when I'm not on the brakes...just dropping into neutral. When idling, although it's smooth, the purring sound "cuts out" very briefly and every so often... Which has me scratching my head and thinking maybe one bank is possibly intermittently dropping?
Three years ago one of my original Digiplex 805 A on my 84 QV US went bad so I decided to just replace both with the Euro QV Digiplex 803 A And it has been working great. Ian, I believe the 801A is not correct for your 85 QV Image Unavailable, Please Login
I had the same problem. It turned out to be the mixture - pretty easy to check, but get a good gas analyser. Cheap ones can drift a bit.
Interesting. was the mixture too rich or too lean? It runs amazing, doesn't smoke or smell rich. Plugs look normal too.
Bill I noticed when I pulled the vacuum hose coming from the Digiplex, the rpm goes up and the ignition timing went advance by a few deg. I don’t exactly know how much but that’s just what I noticed.
The 801 DigiPlex boxes are for a 2v injected NOT a QV....(There are timing curve/advance differences) The correct DigiPlex's for a QV are 803A's for a Euro/ROW and 805's for a USA Market car. Grim
Thanks Grim. So I guess the big question is how would running the 801 vs the 805 differ? From what I see about the curves, idle is the same, but the 805 has 37 degrees of advance @3K rpm and the 801 has 32 degrees. Would it only impact the power band?
Had a similar issue - replaced the vacuum hose that goes into the plenum (picture above) - the rubber was pretty dried out. Problem has essentially gone away but in very rare occasions, the RPM drop so low to the point of stalling - but has not stalled yet.
Update... I replaced the rear bank (11 o'clock) crank sensor and the car hasn't stalled since - about 50 miles. Given the intermittent nature of the problem, I'm not declaring victory just yet, but I'm hopeful. BTW...changing that sensor is one of the most frustrating jobs ever. A lot of posts say to leave it to a shop...and they're right! It taunts you because it's juuuuuust out of reach. I removed the oil breather to gain more access, but it still was a beeotch. Just no easy way to get at those two stupid nuts. That, and one of the studs backed out with the nut, which made putting it back in even more challenging. Glad it's over.
