guess21dan

Guys, I have a 1991 928 which won't start. The engine ran perfectly fine until a small back fire took place recently.

I have replaced the LH, EZK & fuel pump relays, but this still hasnt solved the problem.

Any ideas as what it could be causing this issue. MAF?

Dan.

John Speake

Hi Dan
A backfire can kill the MAF, but the car should still start in "limp home" mode.

Check for spark first...... let us know.

mcholdfast

Where are you located? There may be someone local that could help diagnose the problem on site. There are a lot of folks here that enjoy getting their hands dirty under the hood of a 928.

Bill Ball

Is there any evidence of combustion at all during your efforts to start the car (i.e., does it fire occasionally or not?) I assume it cranks at full starter speed and the battery is good.

Remove the air filter box and check the MAF and the MAF boot. The backfire may have blown the MAF out of the boot or blown one of the breather lines out of the connectors on either side of the boot, creating a grossly lean condition that could prevent starting. It is hard to see all this plumbing - removing the fuel rail covers helps.

After that, it would be best to start with the old general approach of determining whether the problem is fuel or spark before you replace parts. Pull a few spark plugs (one from each distributor/coil), put them back in their boots and hold the tips against the cam cover to see if there is spark during cranking. Or you can use an inductive timing light on several of the wires while cranking, but I prefer direct observation of the plugs. It gives yiou a chance to see the condition of the plugs and whether they are totally dry, wet with fuel, which can be a good clue. If there is spark, put them back in and try starting the car with a little carb cleaner or starter fluid squirted down the MAF throat - not much, just a few short puffs. Some people regard this as risky, but it has always been very helpful as a diagnostic test for me. As long as you don't over do it, it can be very helpful in just a few seconds. Anyway, if the car starts briefly after a few puffs into the MAF, then you have pretty much nailed that you lack fuel. Figuring out why will take some more work. You have replaced the LH, EZK and FP relays. I usually make jumper wires for these 3 relays for diagnostics after verifying that the post 30 connectors are providing 12V. Also, verify that the fuel pump fuse (38) is good. If you jump the FP relay posts 87 to 30 in the CE panel, the pump should run constantly until the jumper is removed. Does yours do that?

Anyway, there are a bunch of ways this could be approached. It would be good to get some direct assistance for another experienced local 928 owner, as Mike recommended.

John Speake has already made some initial suggestions. Here is a link to his further comments on "No start" troubleshooting.
http://www.jdsporsche.com/fault%20find.html

John Speake

Thanks Bill for your helpful comments and suggestions. Dan in here in England and we are also swapping emails.

Dan, I hope this will encourage you to become a Rennlist member, it is a great resource.

tailpipe

timing belt ?

Bill Ball

Mrmerlin

check the timing before you try to turn the engine over with the starter.
Hand crank the engine to TDC

dimmer

I recall a backfire when the computer went on my car ... the injectors let in fuel out of sequence and can result in a backfire. The telltale sign was that you could hear the injectors clicking away like crazy when you turned the key to the ON position (just prior to engine turnover). If it is the computer, there are guys that will rebuild it ... JD did mine a few years ago and I am still happy with the results.

guess21dan

The saga continues!

Ok, I have replaced the EZK/LH/fuel pump relays, that didn’t make any difference.

I then removed the MAF and sent it off to John at JDS for checking. This proved to be working OK.

I have also done a few basic checks for spark at the plugs etc and the spark is good & strong. I’m now thinking that it could be a fuel starvation problem? Fuel pump problem?

I know that LH failure is common on latter 928’s, but I’m not getting the injectors clicking’s like crazy when the ignition is turned on.

Does anybody know where the fuel pressure test point is on the fuel rail?

Running out of ideas now!!!!!

Landseer

Did you jumper the fuel pump relay to determine if it runs?

Fuel pump fuse?

Fuel pump can get power, but also jam internally. If so, hard to detect unless its real quiet and you can hear the hum.

Do you have 12 volts or close to it across the battery terminals?
How about elsewhere in the car? (thinking about ground wires and strap)

Bill Ball

I gave you some ideas already on determining if fuel is the issue.

If you take the cover off of the right side fuel rail, you will find a fitting on the front end that can be used for a fuel pressure gauge. Careful when removing the plug - a large sealing ball bearing will fall out.

If you jumper the fuel pump relay socket (30-87), does the pump run? IF so, you could just lightly crack open the fitting and see if fuel squirts out.

John Speake

Hi Dan
Have you check cambelt timing, as suggested ?

Did you try the starter fluid trick ?

When you crank the engine, does the rev counter (tach) flickker ?

fraggle

He's got spark. That eliminates the impulse sender.

WallyP

While the fuel pump will not be affected by the backfire, it is one of the most common causes for a no-start condition.

Get a helper. Kneel by the rear bumper and have the helper operate the starter. Listen carefully for the whine or buzz of the fuel pumps. If there is no fuel pump noise, check the fuel pump fuse carefully. Check for 12 vdc on the little metal tabs on the visible face of the fuse - there must be 12 vdc on both of the tabs.

If the fuel pump runs, recheck for boots or vacuum lines blown off by the backfire.