MGW-Fla

Ok, I need some direction in resolving a difficult starting situation on the '91 S4. I am just in the preliminary stage of things, so I really would like ideas of the order of most likely culprit to address.

I've recently gotten the car back from my mechanic from a TB/WP job. He's worked on all of my 928s over the years & only works on high end cars. I ran this by him & he immediately gave me his answer. I also ran by him some of my feeble thoughts on the possible problem & he went over each & told me why that would be not the cause. I trust his knowledge from the past dozen years of knowing him, and in almost 10 years of 928 ownership, he knows our cars very well. I'd rather refrain from saying what he suggested is the problem as to no taint any of the 928 experts here. I will give his answer though once I see what everyone thinks.

I don't want to just take the car back to him if it's something I can fix, yet I also don't want to be chasing any rabbits down dark holes with my limited mechanical abilities and even more limited time. He's smart, but no one is perfect. I've searched & read threads here so much I feel like I'm back in college doing a research paper. No single answer seemed to hit it exactly on the head, but I did learn a lot! I know I'll likely get 10 thread links with the exact cause & cure, which would be great. Never said I was any good at college research papers btw!

Ok, the issue. After encountering this issue, a week ago, I replaced the 3 1/2 yr old battery & even keep the new one on a Battery Tender at night just to ensure that I have a fully charged battery. When cranking the car, it turns over fine, eventually stumbles a little bit trying to start, but it never will fire. Turn the key off, wait a few seconds, depress the pedal all the way to the floor, turn the key again. After a couple seconds of turning over, it starts to fire, runs very rough, blows out smoke from the exhaust from all the extra fuel being burned off. If I take my foot off the pedal, it dies. If I keep my foot on the pedal & run the tach at 1500+ for a minute(I timed it yesterday), it will hold idle & run like nothing is wrong. Once its cranked & running, all appears normal other than the 1st couple of minutes killing mosquitoes from the exhaust smoke while its burning off fuel. I've been able to crank it like this the past three mornings & have let it run a bit. If you turn it off & back on, it will crank right up without hesitation.

What is the most likely the cause of this to be addresses? I have tried a few things I read in my search, but so far nothing resolved it. As much as I wanted to resolve this without posting it here, I need some specific direction at this point. My mechanic's answer involves a bit more work, but not something beyond my skill level & I've done it before with past 928s. Its not even that I'm doubting him, I just want to ensure that is indeed the problem prior to me going that route. I did try one thing that I thought is supposed to indicate what he says is wrong, but that yielded no change in the results.

Thanks in advance for everyone's input & help; sorry for the roundabout way I've presented this.

Mel

Mrmerlin

OK so whats happening is that the engine is flooding so a few ways for that to happen the fuel .

Could be injectors are leaking.

The fuel dampers and the FPR are leaking
so lets go easy to complex.


pull the vacuum lines off the dampers and FPR smell for fuel if your smell fuel then I suggest to replace all 3 of these parts.

leaking fuel injector will require them to be sent out for cleaning or swap them for new injectors

mdkelly1

Hi Mel,

What Merlin suggested solved a hot/warm start issue on my car. Your problem sounds like it usually occurs at cold start, but it's still a good place to begin trouble shooting.

soontobered84

Temp II can make it hard to start first thing, then start just fine after the engine is warmed up somewhat, but it shouldn't make the initial voluminous smoke that you speak of. Does the smoke smell of fuel or oil? If it smells of oil, it could be oil leaking around the spark plugs inside the cam covers causing them to foul until it starts and burns off (the smoke).

I think Merlin has the right idea, though

DeWolf

Just bought an 89 and it has the same issue. When it fired it was leaving a fog cloud of black smoke. I got it running and sprayed maf cleaner around the front of the manifold and the engine died. I got my wife, who smokes, to choof on a cigar and blow smoke back down an intake vacuum hose after taping off the maf. A big plume of smoke came out from under the manifold. Pulled the manifold and found a hose that was almost spilt in half. Ordering a bunch of new hoses.

MGW-Fla

Thanks Stan. Does it matter when to pull the vacuum lines on the FPR & dampers to do the smell test? Prior or after starting?

MGW-Fla

Thanks David, so yours was the FPR & dampers to solve your warm non start problem?

Thanks for the info John. I considered the Temp II causing the cold non start, but ruled it out. I'd say the initial smoke smells more of gas & it is white'ish-blue maybe, don't think I've seen any black. I'll go through the process to see for sure what color the smoke is.

MGW-Fla

So your initial smoke was black; What hose did you find was split under the mainfold?

jcorenman

I am thinking that the MAF has gone TU. They normally get leaner with age, but if something goes wonky then all bets are off. Clearly too rich on a cold start, perhaps until things warm up a bit and O2-sensor goes to work.

But start with simple/cheap stuff: Pull the airbox and disassemble and clean the grounds on the engine block, under the fuel regulator and damper.
The LH and MAF ground there, and a corroded ground could definitely cause something like this.

Also check the FPR and dampers for leaks, as mentioned above-- remove and sniff the vacuum lines.

Split hoses under the intake would tend to cause a lean mixture, lots of unmetered air means less fuel than needed so I am not liking that theory for this case.
Leaky injectors are possible, but any fuel that leaked out overnight would be gone by morning. Possible, but unlikely I think.

Temp-II is possible, and cheap to check-- do the resistance check described in the WSM, at the LH and EZK connectors. O2-sensor is not a factor for cold starts.

What happens if you disconnect the battery for a moment? That resets the LH adaptation, my guess is that it gets worse.

So once you rule out the cheap stuff... Can you borrow a known-good MAF? Disconnect the battery to reset the LH.

DeWolf

It was the front right cam cover hose that goes under the manifold to the oil filler base. Not only was it split wide open it basically fell off the oil filler and left part of the hose and the clamp behind.

MGW-Fla

Ok, Jim mentioned what my mechanic said it is, the MAF. And I do have one in my '85 that Injection Labs just recalibrated, so maybe I should of asked if they had a multiple MAF discount. I was planning on switching it out to see if that was the cause before just sending the suspect one off for repair. Just didn't want to go through pulling the one off of my '85 to put in the '91 if that wasn't a likely cause(with apologies to my mechanic). Even though I know that's a simple check.

I'll go through the other easy checks today to rule things out. Temp II was high on my list, but what the heck do I know! The battery was disconnected last weekend when I replaced the battery. Should I do so again since its been cranked? Is there any particular problem with the car that can cause the MAF to go out, or is it typically an age thing? I know if the wire gets damaged its kaputs. Thanks.

Mrmerlin

As your in a guessing game now with lots of great advice,
lets try to work methodically.
First do the fuel smell test. If no smell then lets check it off the list. easy
Check wire connectors to MAF temp2 and any other sensors and put deoxit on them. easy
If you have another MAF swap it out . easy
NOTE disconnect the battery before you start it with any new to the car electronics. easy

Add a bottle of techron to the fuel as well as 6 oz of MMO to the fuel to clean the injectors easy.

MGW-Fla

I should have some time freed up this afternoon to go through the easy stuff. On the Temp II, anyone offhand know the range the Ohm meter should read to indicate its operational? I can search the wsm later today. I thought I read in my thread searching that if both wires have similar resistance its likely good. Already have the Techron in the tank, that's a staple with every fill up in all of my Porsches. What is MMO? Is that Marvel Mystery Oil? Thanks.

mdkelly1

Mel,

Like everything else on these cars, I wish I could say with 100% certainty that replacing the dampers & FPR was the fix for my warm start problem.

This was the classic situation where you drive the car for a while, then park it for 45 minutes to 1.5 hours, then try to start it again. Always resulted in extended cranking and foot to the floor, then feathering the throttle until it cleared the excess fuel (black smoke out of tail pipe). Only took a minute or so to get it to run right once it started after the extended cranking.

That doesn't sound like the cold start issue you are experiencing.

My course of action was:

1) checked temp sensor II - Okay
2) replaced dampers & FPR (all three were original) - problem still there, but less so.
3) changed out MAF (original) - seemed to fix problem temporarily
4) had injectors cleaned (original and extremely dirty with significantly reduced spray patterns) - seemed to fix problem permanently. It still cranks a little longer when warm, but nothing like before.

So, which was it? Probably a combination of all of the above except #1. It speaks to "looking at the usual suspects" first and replacing those items that are known to fail over time.

jcorenman

Here's the info from WSM, resistance depends on temperature. If you check at the sensor, be sure to measure from each terminal to ground (engine block), not terminal-to-terminal. Both can be equal, but wrong, if the threaded connection to the block is not making good electrical contact (installed with teflon tape for example).