Engine misses when throttle is applied. Help.

[jeryst]

I havent driven my 1983 944na in a while, but I took it out last week, and there seems to be a problem when I give it the gas. It stumbles, or misses, like it isnt getting any fuel, but once it get through it, it winds up smoothly without a hitch. Just for clarification, the motor doesnt die, it just stumbles or gurgles.

Here are the specifics...

1. The fuel pressure regulator is about 6 months old, and only has 500 miles on it.
2. I just replaced the fuel filter thinking it may be the problem.
3. I added some dry gas and fuel system cleaner to clean up the fuel system in case there was condensation or maybe bad gas.

4. It does not seem to do it when the engine is cold, but as the engine heats up it starts to happen, and it gets worse as the engine warms up.
5. It does not do it if I apply the throttle slowly. Its only when I try to accelerate quickly. It does it if I hit the throttle quickly at any speed.
6.Normally the car is easy to start, but occasionally, it is very hard to start. It does eventually start however.
7. Like I mentioned above, once it gets through the stumble, it revs and pulls normally.

I noticed that it looks like there are 2 fuel pressure regulators on the fuel rail. What is the second one for, and could it be the cause of the problem?
I've also been told that it could be the mass airflow sensor, the throttle positioning sensor, or the fuel pump. I dont want to start replacing a lot of things needlessly, so any help anyone can give would be appreciated. I have a possible buyer for the car, so I need help as soon as possible.

Thanks in advance for any help you can give.

[johnkoawood]

Originally Posted by jeryst

I havent driven my 1983 944na in a while, but I took it out last week, and there seems to be a problem when I give it the gas. It stumbles, or misses, like it isnt getting any fuel, but once it get through it, it winds up smoothly without a hitch.

Here are the specifics...

1. The fuel pressure regulator is about 6 months old, and only has 500 miles on it.
2. I just replaced the fuel filter thinking it may be the problem.
3. I added some dry gas and fuel system cleaner to clean up the fuel system in case there was condensation or maybe bad gas.

4. It does not seem to do it when the engine is cold, but as the engine heats up, it gets worse.
5. It does not do it if I apply the throttle slowly. Its only when I try to accelerate quickly.
6.Normally the car is easy to start, but occasionally, it is very hard to start. It does eventually start however.
7. Like I mentioned above, once it gets through the stumble, it revs and pulls normally.

I noticed that it looks like there are 2 fuel pressure regulators on the fuel rail. What is the second one for, and could it be the cause of the problem?
I've also been told that it could be the mass airflow sensor, the throttle positioning sensor, or the fuel pump. I dont want to start replacing a lot of things needlessly, so any help anyone can give would be appreciated. I have a possible buyer for the car, so I need help as soon as possible.

Thanks in advance for any help you can give.

My bets are vacuum leak or dead Idle stab valve...

the second barrel on the fuel rail is the damper... pull the vacuum hose off it, if fuel comes out, it is bad.. no fuel and its ok!

[jeryst]

Where is the dead idle stab valve located, and is there a way to test it?

If I would want to replace the dampner, is it called that, or is it another fuel pressure regulator?

[Cocacolakidd]

Have to agree with John on this one - Probably need to clean/repair the Idle Control Valve (ICV), and it is located under the intake manifold. It has two vacuum hoses going into it, one on each side, then it is about 1 1/2" diameter, and 3" long, down to an electrical connection on it's bottom. Hard to get to without taking off the intake manifold. Spray carb cleaner through it to clean, or replace with a good known unit.

First though check for cracked/leaking vacuum hoses - Caution - one can spray starter fluid around the suspected vacuum hoses to see if the motor speeds up if you spray a leaky spot - caution - starter fluid is flammable and used with caution.

The fuel pressure regulator is different from the fuel pressure damper. The fuel pressure regulator is just that, it keeps the fuel pressure at a constant pressure/flow rate, and stabilizes the running of the motor. The Fuel Pressure Damper also helps keep the fuel flow rate at a constant, but as the injectors are constantly opening and closing, spraying, this bounces the flow rate in the system, and the Fuel Damper smooths out all of those pulses to keep the system even and reliable.

[nynor]

as above, or a bad track on the AFM.

[jeryst]

Is the missing, and the hard starting related?

The hard starting only happens occasionally, but it started when the missing started.

BTW, the car idles fine and pulls smoothly unless the throttle is applied quickly and the engine is warm. Is that definitely how the ICV would act if defective?

I'm not very mechanically inclined so I would need to have someone do the work for me, and I dont want to go through the expense of having someone tear off the manifold and replace the ICV unless I'm 90% certain that is the problem.

[Cocacolakidd]

the hardest thing to do is to actually work on a car that is 800 miles away - Some of us who have experience with our 944's can only give our best guess advise...What you really need to do is to take it to your mechanic and have him diagnose the problem when he is actually there with the car and can see what it is doing when it is doing it.

[jeryst]

Originally Posted by Cocacolakidd

the hardest thing to do is to actually work on a car that is 800 miles away - Some of us who have experience with our 944's can only give our best guess advise...What you really need to do is to take it to your mechanic and have him diagnose the problem when he is actually there with the car and can see what it is doing when it is doing it.

Unfortunately, there is no one around here that works on them, other than a few other 944 owners that have done their own repairs. They dont have any diagnostic equipment or anything like that. None of them are enthusiastic about tackling any major repairs on other peoples cars, although they will offer to help out a bit. I have a good friend that is a mechanic, but he has no experience at all with Porsche's, so he really doesnt know what he is doing either.

[Cocacolakidd]

Well it seems like it's time to buy some wrenches.

Start with the simple easy things first.

With caution start with starter fluid and spray the vacuum hoses with it - if the motor speeds up while doing this you have a vacuum leak.

Check the connection at the throttle body for the throttle position sensor - big plug going to the throttle body - clean both the male and female contacts.

If none of the above check on the ICV (Idle Control Valve)

[bell]

fyi the coolant temp sensor for the dme can cause the same problem, chased one in a 951 for a couple weeks.....as the temp would start to change the sensor would fail then work, then fail.....check the connector for corrosion etc......

[mytrplseven]

I'm a proponent of a different tack for your temp sensor and vacuum (could be connected) possibilities. I'm not a fan of spraying volatile stuff into running engines. I'll attach a pic of an approach that is safe, cheap and works every time. You'll need an air compressor, a 2" pvc cap, an air nipple that can be threaded into the cap and a spray bottle of soapy water solution. Apply 7 to 10 psi onto the engine components, including the throttle body, under the intake manifold (especially 2/3rds the way back where the temp sensor vacuum lines connect. There's one in and one out there), the lines to the fuel pressure regulator, fuel pressure damper, all the vacuum lines attached to the throttle body and from under the J boot. there's a diagram of vacuum hose routing on Clark's Garage to help you find all the fittings. Also check the connector to the AFM and TPS for clean contacts and the Idle Stabilization Valve (under the intake manifold, spray vacuum lines in there, too). Any vacuum leaks will show up as bubbles (see the throttle body pic).

[Aufenbahn]

Another thing nobody has mentioned is your O2 sensor - I chased a similar problem that started small and eventually made the car almost undriveable - fixed it finally by replacing the O2 sensor.

[GreenWater]

mytrplseven, good idea, I never thought about doing it backwards. I am one of those, "spray carb cleaner on the part to see if the idle goes up", guys. Your idea is pretty good, I will have to remember it for when I need check something.

I do like the danger and thrill from spraying carb cleaner on a running engine. haha

So many things to check, so little time.

[jeryst]

Well, so far I have replaced the fuel filter and fuel pump, and there are no vacuum leaks.

Is there any way to check the Idle Control Valve or the Throttle Positioning Sensor? I dont want to keep replacing expensive parts.

[HondaDustR]

The throttle position sensor is easy to check. Look here and click on the link that says throttle position sensor -> Clark's Garage Shop Manual Index

Make sure the switches are working using a multimeter set to ohms or continutiy mode.

An easy way to rule out the AFM and o2 sensor is, once you have verified that the TPS is working correctly, try to replicate the problem. If the stumbling is present but disappears when you go full throttle, i.e. activate the WOT switch in the TPS, the o2 sensor or the AFM is definitely possible causes, since WOT puts the DME into open loop mode and ignores the 02 sensor and the AFM signals. This will also help determine if significant vacuum leaks are causing problems, since normally, vacuum leaks will make the AFM read low, resulting in less fuel delivery and a lean mixture, but in open loop mode, the DME doesn't care where the engine is getting its intake air. If WOT doesn't affect the symptoms, it's likely something else other than the AFM or the 02 sensor (electrical supply wiring problems/corroded grounds, fueling, ignition, crank sensors (although they usually either work completely or fail completely)), but you should still check the AFM and also try running with the o2 sensor unplugged to troubleshoot. What rpm does the motor straighten out? I had much the same problem on mine that would show itself when the car warmed up. It would instantly run fine as soon as I hit WOT, and would also run fine with the 02 sensor unplugged. A new O2 sensor cured it. Also a good idea to break any corrosion in all of the electrical connections for the engine sensors and the DME by unplugging and plugging back a few times.

Also check the DME temp sender and the intake air temp sender (in the AFM).

I think there was a member here (texasblake IIRC) that had a basic checking procedure for the IAC valve, but I believe the valve uses PWM signalling and would be difficult to actually test thoroughly without complex electrical equipment.

My next question if the above investigations are inconclusive is what is your actual fuel pressure and pump delivery rate?

[gtroth]

I had horrendous hesitation when it was very cold out and engine was warm. Wouldn't describe it as stumbling exactly, just push the gas and nothing, until it hit and accelerated some. Undriveable that way really.
All AFM voltages (temperature and door response) measured fine every single time I measured them.
Nevertheless, it was the AFM for me. New one and problem solved.

[jeryst]

I'm located at zipcode 15801. Western/Central PA, near I-80.

Anybody near me have a parts car that they would want to sell the TPS, and AFM from?

They are going for pretty cheap on ebay, but I prefer to buy local, especially if a fellow member can make a few bucks.

I would simply try each piece individually, and buy whatever fixed the problem. Win-win.

The reason I would like to do it this way, is because I dont have a garage or test equipment, and the car is sitting outside. Easier just to swap out a sensor and see if the problem is fixed.