Hello all,
I recently picked up a 951 and the seller had remarked that it misfires at wide open throttle.
Upon driving it though, it misfires badly at any throttle input besides a light touch, 10% or so. It backfires, pops, jerks and bucks.
I gave it a tune up with NGK BPR6ES gapped at 0.028", new Beru wires, and new Bosch cap and rotor. Cap is a PITA to replace. It didn't make any difference in the running condition.
The AFM seems to operate properly. The previous owner's mechanic must have been through it at some point because the top of the AFM had been removed.
If I raise the throttle with my right hand and my left hand guiding the wiper arm of the AFM the misfires seem to disappear and the car will rev all the way to wide open.
It's almost like the AFM is delayed in it's actions, causing the car to be lean? I don't know, just supposing.

Also, FWIW, the vacuum lines look recent and don't exhibit any failures.
There is a rushing air noise near the AFM, maybe from underneath it?

Any help or advice would be massively appreciated.
Cheers!

 

is his the white turbo that was for sale in los altos?

 

It's a black turbo with 911 wheels. It was purchased in Los Gatos / Monte Sereno area.
Ivory/Black two tone interior.
Are you familiar with the car?

 

IIRC this was a clean car with high mileage, otherwise i am wrong, but based on the evidence of behavior, I would search for a good used AFM that has not been tampered with, but still check the basics which you already ruled out ignition as the problem seemed to diminish when sweeping the afm. the only other thing to consider is a clogged fuel filter or collapsed fpr or fuel damper; however, I would not expect the afm sweep test you mentioned to matter if it was a fuel supply issue. another simple test would be to wiggle the speed and reference sensor connectors while attempting to rev the engine past 2100 rpm regardless of load presence and also to check the ground on the cylinder head for corrosion / corroded battery ground wire not supplying good ground to input/output sensors.

 

@kev951 Thanks, I'll do that tonight when I get home or tomorrow.
You are correct, 200k miles but in decent shape. The body and interior have several minor flaws but overall acceptable. The part that appealed to me was that it was the original owner that was selling it. And I'm a sucker for the ivory/black two tone interior.

A good used AFM is about $500, which in my mind isn't far from the LR MAF conversion. I might be justifying a little, but it's an upgrade and rules out the AFM as an issue. If I can check through some of the other failure points you described and everything else checks okay, that might be where I go next rather than replacing with a used AFM. I could be accused of trust issues but I find it hard to trust the retailers of used parts..
Justin

 

I was looking at that car but the buyer just wanted too much at the time. I mentioned clutch, rod bearings, rear spring plate bushing replacement, but they wanted none of that during negotiations. you can go the MAF route, but the Lindsey kit is pretty outdated and not great for tuning and you will have to deal with failing a visual smog check every 2 years. Vitesse maf kits from John are great and support still exists. Rogue mafs and ecus are awesome but there is not more support for Josh's products. I would just keep things fresh on that car, and leave the engine bits OEM, I am in San Mateo if you ever want to chat or meet up for a drive on hwy 9 or 35 or 84 don't hesitate! but ... I won't be in my 951 as it is still in major overhaul resto-mod phase (wiring, plumbing, paint and body remain) and I am looking for a turn key daily driver 951 not needing anything mechanically and will sell my subaru sti daily driver

But one more thing to check functionality on is your TPS. a simple check with the engine off is to crack the throttle open by hand very slightly, an audible click should be heard, followed by a second click when closing the throttle. Just another sanity check.

 

@kev951 I won the car at BaT auction. It does seem like it needs a clutch, you thought so too? Rod bearings and the rear spring plate bushing replacement? You're not thinking con rod bearings, are you?
Did you drive the car at the time you were considering buying it? How did it drive? When I picked it up it was bucking so hard that I couldn't drive it home.
I don't have to worry about the biannual check once I pass the initial inspection. I have a house in a county where it isn't required unless changing ownership.
I'll check the TPS as you described. Vitesse might be the way to go then.

There's a nice looking '88 951 in Sparks, NV for $22k. I haven't seen it but it looks clean and has 115k miles.

I would love to cruise, I'm based out of Monterey Bay these days. I'm working on putting together the engine for my 912 and also trying to 6-speed manual swap a 957 Cayenne GTS

I'd love to find a 944 that I can put Turbo fascia on and drop in an LS3.

Your help is invaluable in saving me time and headache! Thank you

 

@kev951 I heard the click when I raised the throttle from closed but I did not hear a second click at WOT

 

@ACBSea the seller would not let me test drive it due to our negotiations. Yes, rod bearings and clutch are concern on that car. Also, 100% of any 951/944 you find, unless stated with proof by the owner will need rear spring plate bushings as this is hardly every addressed but is quite critical for optimum suspension performance and strange rear geometry issues. those three services alone are a huge slap to your budget if looking for a "car that needs nothing" when stated by sellers. What's even more concerning are these cars were super low miles being all original including the fuel lines.. I mean do you trust 30 plus year old used rubber? Basically I got the " I know what I have and could care less of what you know you think" type of response from that seller. For the TPS , it's one click when opening the throttle very slightly , and a second when closing the throttle. The next thing to check is the actual actual TPS sensor resistance values and WOT contact: https://www.clarks-garage.com/ > FUEL-06, Throttle Position Switch - Information, Troubleshooting, Replacement, and Adjustment

Sidenote - I do come to Marina about once a month during the week on wednesdays and thursdays for work for the day. Usually grab a sandwich from Grinderz Deli aka The Local Deli
Otherwise I work in Santa Clara / San Jose

 

I checked the TPS and AFM using the Clark’s-Garage write up. Both checked within spec. TPS is a bit suss, but even with slow movements it doesn’t exhibit flat spots/erratic readings. I checked the connectors at the DME and KLR and they’re looking good.
I also put a fuel filter in despite the one in there looking recent. I just don’t know what’s good and what’s not.
I’m considering opened the AFM and moving the black dial to richen the mixture, after marking its existing position so I can return it to where I found it if it doesn’t help.

I’m not out of ideas but I’m frustrated with the car. It’s miserable to drive.

also, rear spring plate bushings look shot, as expected. Clutch functions for now but the catch point is all the way at the end of the travel, which normally means it’s at the end of disc life

 

@kev951
I’ve never tried Grinderz but I don’t ever go to Marina. I’ll have to take my wife over to check it out.
I like Randy’s Deli in Monterey at 10th street. Or Compagno’s in PG. Otherwise, best food in the county is always at home: my wife is a Cordon Bleu graduate with impeccable taste
I had the best clam chowder for dinner tonight, that woman deserves a medal

 

Awesome!!! 🖤🖤🖤

 

Before you mess with the AFM can you post a picture of its current state to see if the tamper hole is still intact and to see if there is signs that the cover may have already been removed? Also if the car is idling fine just for s**** and giggles wiggle the speed and reference sensor connectors and see if the engine starts to stumble when doing so. But I'd be willing to bet that you have a failing TPS , if you unplug it are there any signs of oil in the connector itself? Otherwise the switch can also fail when there is excess oil in the charge tubes aka intercooler pipes.

 

The TPS tested good (doesn’t necessarily mean it’s all good) according to the Clark’s Garage walkthrough on the electrical readings. I triple checked the open/close click on the micro switch. It seems to work, key word being “seems”. The readings are smooth and not erratic when I checked very slow movement of the throttle against resistance from DME pin 2.
The plug was dry, no oil. I haven’t removed the intake boot in front of it to see what the throttle body looks like inside.

I don’t know if this helps. But that’s the state of it when I picked up the car. I removed the intake snorkel piece and the black AFM lid was sitting loose on top. Someone had been into it and it may have been recent.


again, your help has been invaluable to me. I appreciate it a bunch. Owe you a beer and a sandwich, hey!

 

Do you think DME temp sensor could be an issue? I just re-read Clark’s-Garage instructions and I was meant to unplug that DME temp sensor wire when doing the test and I didn’t.

I might try to adjust A/F ratio with the big black wheel inside the AFM as a Hail Mary…