Zachattack8805

I have been a long time lurker of this forum but this is my first post so bear with me. I have a 1990 GT which has ran great ever since I got it in 2016. About 6 months ago I went to start it after I had gone for a long drive and it was still warm, it tried to fire once and then just cranked and cranked continuously with no sign of life. I gave up on it and re-visited a few days later. This time it fired up no issues and drove great. Months of regular use went by and the car had no issues until the same thing happened again. after it started happening more frequently I wanted to actually solve the problem. Here is what I have tried so far:

Compression test checked out
Ignition is working as I have spark
Fuel pump is running as I definitely have fuel
Injectors don't click with ignition on like some have reported with a bad LH computer
cleaned ground connections
replaced battery (needed it anyways)
Replaced Crank Position Sensor


So after noticing the spark plugs had fuel on them I came to the conclusion that the engine is flooding. I confirmed this by pulling out the fuel pump fuse and cranking with WOT. Sure enough it started to sputter to life after cranking for a few seconds. This however is where things get confusing for me. With the fuel pump fuse out the engine runs rough but will continue to run for a long period of time. I'm talking like 5-10 minutes and probably longer I just decided to shut it off at that point. I don't understand how the engine is getting fuel to run when the fuel pump has no power. The other weird thing is while the engine is running rough with the fuse out if I pop the fuse back in it briefly runs smoother but then dies and goes back to crank but no start. My theory is possibly a bad fuel pressure regulator? I would just swap it out to see but for '87 and up cars the OEM part is something like $600. Also I am partially skeptical of the FPR because it doesn't seem like there is any fuel in the vacuum line coming off of it (only slight fuel smell), and it seems like the car would still at least try to run not just immediately flood. Curious if anyone has experienced anything similar or has a good explanation as to how the car can continue to run with the fuel pump fuse removed. Any help is greatly appreciated, I'm dying to get the shark back out on the roads while the weather is getting good.

Tom. M

Temp II sensor on the water bridge? Mine did the same, ran great on start up. Couldn't restart and had fuel coated plugs..
There's a testing procedure somewhere posted on Rennlist...

Zachattack8805

I will check this out, thanks!

FredR

Our engines cannot possibly run without the fuel pump spinning and developing pressure so perhaps you need to physically check as to whether the pump is running rather than rely on the inference because the fuse is out. Previous owners have been known to do strange things and quite a few folks have configured fuel pumps with new wiring setups although doing so with the fuse completely out of the loop would be bizarre to say the least but if it was independently fused no real problem.

It sounds as though your starting problem is with hot starts and the first thing I would look to eliminate [if that is the case] is the temp2 sensor- just check the hot resistance it is showing either at the computer contacts or on the sensor but remember the sensor has two channels - one for LH, the other for EZK and to check resistance on the sensor make sure you do so measuring resistance of each pin to earth- not across the two pins. If you have a failed temp2 with cold resistance values the motor will be overfuelled by some margin and thus may explain wet plugs. Also, if the system is not holding fuel pressure [leaky injector or failed check valve] when the engine stops running the heat can cause the fuel to vapourise in the rail and then starting is difficult or nigh on impossible until things cool down- I have had this happen at fuel stations- if tha tis the problem open the bonnet to let hot air escape quicker but you would still need to determine the root cause of the problem.

You should also remember that with the LH whereas there are common failure modes there are also less commmon modes of failure and remember that is the unit that computes the pulse width for every firing stroke.

I have just had a most unlikely EZK failure- a very rare event- I could see sparks on both firing circuits but the thing would not come to life- changed out for a spare EZK unit and the motor immediately roared into life- always expect the unexpected and suspect the obvious until proven otherwise! Stick you LH computer in another 928 if possible and see if the problem migrates. Remember units like these can also show erratic behaviour until they fail completely. A friend with an 87 S4 had an erratic problem where the car would stop after running a few minutes [enough time to get warm on the circuit board] and then refuse to start. Stuck my spare LH unit on it and it ran perfectly- tthere was nothing wrong with the secondary tile that starts the fuel pump [most common failure] but John /speake nonetheless put a new tile in it after he got it for refurb.

Michael Benno

You may find this general document on the S4 and GT fuel and Ignition system helpful:


But whenever I am troubleshooting engine running, I found this EZK-Ignition and LH-Jetronic troubleshooting guide super helpful in determining he appropriate sequence of tests and helps save time in testing hypotheses.



Page 7 has a super helpful troubleshooting matrix on it.

Zachattack8805

it is not really a hot or cold start issue it just wont start at all, in the past when I have had this issue it usually goes away after a few days and will start maybe run erratically for a few mins and then be fine. How the car sits now though it will not start at all unless I remove fuel pump fuse. then it runs rough until I re install the fuse. I've heard these engines can very easily flood so I'm thinking its just an over fueling issue but not sure what can cause that other then the LH. If the temp II sensor is reading way off would the LH actually change the injector pulse time? So a failed Temp II could cause the LH to max out injector pulses which results in instant flooding?

Zachattack8805

yah I looked through the service manual for awhile but couldn't see to pinpoint one specific problem that explained my situation. I: suppose its certainly possible there are multiple issues at play here. appreciate the pics though.
Also glad to see I'm not the only one with a shark in Oregon!

PC-85-928S

Temp II
-mine did something similar also
-make sure you were pulling the correct fuse

ammonman

Pull the vacuum lines off of the two fuel pressure dampers and smell each one for fuel, a sure sign of a broken diaphragm. When the diaphragms in these units fail fuel is pumped and sucked into the intake manifold, flooding the engine. If you are getting even faint smell of fuel the unit is suspect. Another quick check is the next time the car won't start put the accllerator pedal to the floor and try starting it. If the manifold is flooded with fuel opening the throttle full will usually allow enough airflow to lean out the mix and let the car start.

dr bob

Start with the known/easy stuff --

1) Relay-relay-relay, and sometimes another -relay. Fuel pump, LH, EZK, and the sometimes one is the X-bus relay. First three are required for engine running, the last for the slew of accessories like lights that depend on engine running. Get known-good Bosch 53B relays. Our vendors carry good ones.

2) The injection, ignition, fuel pump plus the cooling fan circuits get power from dedicated wire feeders from the battery positive terminal. Carefully remove and clean each of those smaller red-wire connections at the positive post, looking for shiny metal after a treatment with your small brass detailer's toothbrush. Snug the bolts/nuts, and coat the whole cleaned battery terminal assembly with Vaseline.

3) If you haven't already done so lately, invest the time in cleaning all the ground points in the car. Similar to the way you detailed and dressed the connections at the battery positive, these all get a buff with the brass toothbrush until bright and shiny, and a thin film of Vaseline once they are reassembled and tight.

These three things seem to solve an amazing number of electrical erratica.


4) The LH box has some interesting failure symptoms, and the clicking-injectors is not always apparent. Mine did not have that symptom on failure, yet another local's S4 did. Your discovery of fuel on the plugs hints at either too much fuel (failing LH or otherwise), or maybe intermittent ignition. Try your LH controller in another car to verify that it's OK, and if it's never been rebuilt/updated have that done now regardless of other symptoms. They inevitably fail, and relatively few of us have that happen in our own work bays with plenty of time to solve it. Just Do It.

5) Erratic ignition in S4+ cars is often traced to a weak/failing CPS (crank position sensor) or it's wiring/connections. The same sensor drives the tach, so if your tach doesn't bounce while cranking it's a good place to look. Replacing that is possible without a whole lot of disassembly, but usually when the wiring for that is failing, the other wiring in the area (knock sensors, MAF, and throttle position switch) is similarly due for replacement. For that the intake comes off and all that exposed wiring and connectors get a hard look. Search the forum for "CPS Replacement" for some more guidance.

GregBBRD

Highly likely.that a damper is bad and there is raw fuel getting into the intake system.
I'm making replacement dampers and regulators (rebuildable), now that the Porsche price has gotten extremely high.
You can buy these directly from us or from 928 International.

soontobered84

If it's not the Temp II, pull the spark plugs and check them to see if they are fouled.
I had a similar issue with my 89 and it was the spark plug seals inside the cam covers were leaking oil into the spark plug holes.
I changed plugs for the quick fix, then replaced the gaskets/seals in the cam covers and have never had another problem with it.

Zachattack8805

If a damper or FPR was bad and fuel was getting sucked into the intake wouldn't it be obvious with alot of fuel on the vaccuum lines? I ask cause my vacuum lines are totally dry off of both dampeners and FPR even after cranking for awhile and flooding the engine. It seems like if It was sucking enough fuel past the diaphragm into the intake to instantly flood the engine then the vaccuum line should have fuel all over it no?

Zachattack8805

So from what I read on other forums it seems like if you completely unplug the temp II sensor it will default to a usable amount of fuel. I unplugged the sensor and it made no difference. It is interesting you mention spark plugs seals cause I have had some oil leak into a couple of the spark plugs tubes however not all of them and when I replaced the spark plugs previously it still wouldn't start.

Mrmerlin

did you test the temp 2 sensor?
to test it ,
one pin in the sensor to the red lead,
the other lead to ground,
note the reading in OHMS.

then put the red lead onto the other pin, repeat the numbers should match ,
use the WSM to figure the temp of the engine and the requisite Ohm reading,
both of the pins should be close on ohm reading.
NOTE this is a sensor with 2 separate circuits in it.
NOTE also look at the temp2 connector if the boot is damaged or the wires appear to be shedding insulation,
cut off the old connector then solder new connector onto the wire then heat shrink, its called an AMP 2 pin connector ,
or a Bosch 2 pin connector usually comes with a new boot