7a Hot Start Problem

[RTurner992]

Hi,

I'm trying to solve my hot start issue on my 7a. When cold, it starts perfectly, but as the temperature rises, it struggles more. When it's warm, the only way I can start it is WOT, and it'll run like it's misfiring slightly for a couple of minutes at idle, or return to normal running if I drive it down the road.

I did a pressure test per the Bentley manual between the supply line and fuel rail. I found that as soon as I turned the ignition off, the pressure dropped rapidly. This was a while ago, but from memory, it dropped to about 1 - 1.5 bar within 10 seconds. Whatever the reading was, it was way out of spec. My first thought was the checkvalve at the pump. It's running a 040 Bosch pump, so I picked up a Bosch check valve, and that didn't change the system pressures at all. I bench tested both the original checkvale and the new one, and they both appear to be doing their job.

I'm fairly convinced my pressure tester has failed after using it, as I struggled to get more than 1 bar on the gauge after reinstalling the fuel pump, but it fires fine when cold, and drives fine as well. I need to test my tester or get a new one I guess.

Kind of for the sake of it, I bought a Radium FPR with an interchangable bosch diaphram. After redoing the fuel lines and installing them, there was no change in behaviour. I know the OEM ones are pretty bulletproof, and there were no signs that the diaphragm had failed, like fuel in the vacuum line, but it was just something I thought could be easily eliminated and future-proofed.

When I bought the car in November, I replaced all the coolant sensors, so I removed the G62 sensor and reinstalled the OEM one I previosuly removed. I tested both and although the new Bosch one wasn't as accurate, they both seem to be doing their job correctly. I know I shouldn't have blindly changed things, sometimes I can't help myself haha. Currently, the OEM sensor is installed, again, no change in behaviour.

The sensor for the fan afterrun is also reading correctly, and the fan is turning on and off as expected.

From here, I'm not sure where else to be looking. I did pull the spark plugs, and the threads on one, maybe two, were slightly wet, although I couldn't tell if it was fuel or not.

Any suggestions would be much appreciated. Also, I still haven't got to the bottom of my rattle from the distributor and belt slap, which is getting so discouraging. If anyone missed that thread and is knowledgeable about the 7a, please take a look at my thread - https://www.s2forum.com/forum/genera...out-of-options

Any help with both issues would be appreciated!

[RTurner992]

Anyone got any ideas? Thanks

[Bowie69]

I used to suffer with this, after a bit of a rolling resto I did replace both VR sensors pointing at the flywheel, I have never had the issue since, and I don't think there was much else I changed that could have effected it.

That was maybe 10 years ago now.

[RTurner992]

I used to suffer with this, after a bit of a rolling resto I did replace both VR sensors pointing at the flywheel, I have never had the issue since, and I don't think there was much else I changed that could have effected it.

That was maybe 10 years ago now.

Thanks for the input.

I managed to find time this morning to swap out the G4 sensor and it's made a difference. I've been able to hot restart without WOT, however, I feel like there's too much fuel. It doesn't idle lumpy/ feel like a misfire at idle, but if you blip the throttle after the hot restart there's a huge delay in response. As before, if you rev it up abit or drive down the road the response returns back to normal.

Does this sound like over-fuelling? Are we into leaky injector territory at this point?

[Bowie69]

What are you running for injectors?

Why an 044?

It's possible it is overwhelming the return line and over pressuring the system.

[RTurner992]

What are you running for injectors?

Why an 044?

It's possible it is overwhelming the return line and over pressuring the system.

I'm running the stock Hitachi injectors. I've had them all out, cleaned them, which restored the spray patterns a good while ago. I did it the DIY way with pulsing voltage and spraying injector cleaner. Around that time, I didn't have the hot start issue before or immediately after. The fuel rail was off when I was doing the cam chain and lifters, so I just did it while they were out. I plan to send these off at some point and have them flow tested and ultrasonically cleaned for good measure.

It's the 040, not the 044, which I believe is closer to the OEM spec than the 044. A previous owner fitted it. I only discovered it when installing a new Bosch check valve to rule out the check valve being the issue.

Edit: I didn't experience a hot start issue from ownership in November 2024 up until the past few months. It's hard to pinpoint exactly when it started with the car being in bits for ages trying to sort the distributor backlash and cam belt slap. I do recall the order of whch jobs I carried out though. I'm still in touch with the previous owner, who owned it for 2-3 years. He didn't report having any hot start issues, and it wasn't he who installed the 040 pump.

[RTurner992]

Rereading my post from earlier today about the G4 sensor making a difference, I took the car out for abit this afternoon and it did fail to start again without WOT, so it looks like that wasn't a part of the issue.

[RTurner992]

Ok small update: I got the car to a point where I knew it would restart and pulled the spark plugs. Non of them looked obviously soaked, however, the spark plug hole of cylinder 1 did smell like fuel. The other 4 didn't smell at all. I have 4 spare injectors, so I replaced injector one to try and rule that out but the issue still remains. I haven't pulled the plug yet to see if the hole still smells of fuel. Currently waiting for it to cool a little first.

I genuinely thought, after smelling the fuel, that swapping out the injector would solve it. Any ideas after this update?

[K Simmonds]

I know this would mean much more expense but I though I should mention I fitted an 034 7A upgrade injector kit to one of the Audi 90 7A quattros I maintain for the owner after one of his injectors completely failed and he lost a cylinder. He has three 7A 90's all of which are in great condition, the quattro and two FWD cars one of which is for sale, I had to sort out a multitude of problems with the that one.

Anyway, he got the kit brand new from the states while he was over there a few years ago, it consists of five much more modern injectors and a bit of wiring that need's to be installed to to the module on the drivers side close to the fuse box.

I was somewhat skeptical that the it would all go together with no leaks as the injectors aren't anywhere near as tight a fit in the rail a the the OE ones I removed but fair play it all went together like clock work and is totally plug and play.

It transformed the car, much nicer to drive and no starting issues hot or cold, previously it could be very hard to start hot.

[John.]

The injector swap as Kit has mentioned above could be your answer. Some more info here....... Injectors

[Lago Blue]

Seen this?

In the thread link below for an UrQ hot start problem (which also contains at the end; a link to my 7A G4 fix for hot start issue), two people mention the distributor's hall sensor as a possible cause:

https://www.s2forum.com/forum/genera...em#post1329216

[RTurner992]

Seen this?

In the thread link below for an UrQ hot start problem (which also contains at the end; a link to my 7A G4 fix for hot start issue), two people mention the distributor's hall sensor as a possible cause:

https://www.s2forum.com/forum/genera...em#post1329216

Thanks Lago Blue I did test values of the hall sensor, along with MAF, ISV, G4, G28 and so on to rule those out, however, I didn’t measure these when hot so it’s probably worth going back and checking everything again. I’ve received the injectors (discussed in the other thread) and I’ll be installing those soon although I’m not 100% convinced that’s my issue. My attention has been drawn to the N80 and the check valve on the carbon canister. I checked the N80 and its values, and continuity to the ECU, as well as being able to feel it ‘buzz’, so as far as I can tell that’s operating as it should. I do need to pull the check valve and see if that’s functioning properly. As far as my understanding goes after reading the Bentley manual if that’s failed it would create a hot start issue.

[RTurner992]

Hi all,

I’m still trying to solve this issue. I’ve tested everything (MAF, G4, G28, hall sensor etc) hot and cold and they all check out and are within spec.

I didn’t install the Bosch 040 fuel pump that was installed and I have no idea how long ago that was installed as I have no paperwork for it, so I decided to swap it out for a new one. I went with the Bosch 200 (the Bosch 044 replacement after they were discontinued). I also ran new wiring and relayed the new wiring off the original relay. I’m getting 0.2v drop from the battery to the pump. Lovely.

I also installed the Bosch injectors that I mentioned previously, but unfortunately there’s absolutely no change in symptoms after carrying out all of this work. Although, I can be fairly confident that the Hitachi injectors weren’t leaking now. I also changed the fuel filter incase of a blockage. Not wasted time or money as it’s nice to have new parts installed, but still frustrating that there’s been zero change.

I’ve reverted back to testing pressure at the rail to investigate further. So from cold with a gauge between the supply and the rail, I get around 3.6 - 3.7 bar. I’m not sure whether to be concerned about that as the Bentley manual says “System pressure 3.8 - 4.2 bar”, but let’s continue.

I let the car idle for maybe 10 minutes, so by no means hot hot. Turned it off and did a residual pressure test. I left it for 15 minutes and the rail pressure dropped to 3.2 bar. This checks out, Bentley manual says “Residual pressure after 10 minutes 3.2 bar”.

This is the part that confused me. I cycled the ignition ready to fire it up, and the rail stayed at 3.2, not even a wobble at the needle. I cycled the ignition several times to build pressure but nothing. Usually on a “hot start” I’d have to use WOT to get it going, but I thought I’d let it crank and see if/when it caught. It probably took 8 seconds until it caught (without WOT). Usually starting with WOT the engine has a rough and high idle. If I let it idle for a few minutes everything starts to settle, but this time without WOT it idles closer to a cold start idle. Side note: cold start idle is rock solid at 850rpm.

Back to the rail pressure. After it started I checked the gauge, which was still at 3.2 bar. I let it idle for a bit to see if it would increase, I gave it some revs but it stayed at 3.2. I then pulled the vacuum line from the FPR and plugged the hose (manifold side) and left the FPR open to atmosphere. If I’m correct I was expecting to see the gauge jump to 4 or so bar, but nothing, it stayed at 3.2 bar. Bear in mind, I have changed the FPR to a Radium one with a Bosch 4 bar diaphragm. Early on, I only changed this to rule out a faulty OEM FPR, but there was zero change in symptoms between the two.

I after I turned the car off, I cracked the fuel filler cap and there was a pretty big hiss. I’ve read threads about whether there should or shouldn’t be a hiss. To be honest I’m still non the wiser on that. I couldn’t tell you without testing again whether the fuel tank was under vacuum or pressurised. I tried to start again after and no change to the usual symptoms.

So a few thoughts. I’m clearly having some issue building pressure, not from cold, but after it’s been running for X amount of time. Time equates to heat, and if I’m not building pressure then is this a case of heat soak and the fuel in the rail boiling off? Creating air pockets which in turn restrict building pressure back up?

I don’t think the 3.2 bar is a specific relevant number. I think if it dropped to 3.1, 3, 2.9 bar I’d have had the same issue. The point is I didn’t get pressure build from the residual pressure value.

I have tired variations of unplugging and capping the vacuum lines from the solenoid and check valve on the carbon canister just to try and get a different outcome and the symptoms remain the same.

I’m not entirely sure on my next move here. What’s throwing me is I can get pressure from a cold start (granted it’s slightly below spec), so I’m not convinced it’s a physical blockage in the supply line. I don’t have a fuel leak anywhere I can see. Maybe a split supply line inside the tank? Any further advice on this would be great to try and brainstorm this stage. I’ve changed so many parts so it shouldn’t be that hard to narrow this down surely?

[Lago Blue]

Hi Sir,

As you appear to be proceeding very thoroughly, here are some items you may have already checked, but not yet mentioned:

- Do you a.) have a functional CEL, and b.) have any stored fault codes?

- How much vacuum a.) does your engine pull at warm idle, and b.) how much is delivered to the FPR?

- Is the CCV hose the original all rubber, or the newer metal one; and is it totally intact?

- Does your ISV's (possibly old?) hose collapse during hot cranking?

ISV hose collapse durung hot cranking Screen Shot 2026-01-02 at 12.46.13 PM.png

- What is your battery voltage?

- Is your alternator output steady?

- Do you have spark while the problem is occurring?

Here is another idea:
https://www.audiworld.com/forums/aud...olved-2128088/

[RTurner992]

Thank you Lago for your input again. I hope you had a good Christmas and New Year.

I was questioning the results from the gauge I had been using to monitor fuel pressure, so I purchased a second gauge to rerun my tests. Here are the results, which strangely differ from the original test.

From stone cold:
Ignition on the fuel pump primes for around 2 seconds. During the prime, I get 4.1 bar.
After the pump stops priming, but ignition still on, pressure drops to 2.5 bar.
Cranking, I get 4.1 bar
Idling, I get 3.6 - 3.8 bar (gauge is bouncing around, but might be due to the gauge not being dampened. Previous gauge was rock solid)
Idled for 1 - 2 minutes. Engine off and pressure drops immediately to 2 bar.
Interestingly, pulling the FPR vac hose while idling does make the gauge jump to 4.1 bar, where it did not with the previous gauge.
There was also one instance where the gauge dropped to 2 bar when the engine turned off, but over 1 minute (timed) it rose to 3 bar. Strange. The engine was still cold, having been run for only a few minutes.

This test (other than the instance of rebuilding 1 bar over 1 minute after turning off) has been repeatable from cold. I haven't tried this hot yet, as I ran out of time, but to me, even from cold, there seems to be a sharp pressure drop that shouldn't happen. Trying a different gauge with different results might be a red herring, and I don't want to risk providing too many incorrect findings based on faulty equipment, but I thought it would be worth sharing these new findings. Of the test results, I can't confidently say which gauge I trust more.

As a side note, pulling in 3rd through the revs, there's absolutely no hesitation through the rev range, no missfire, splutter or anything, it's rock solid, so I don't think fuel delivery or a vac leak is the issue. I've fixed many vacuum leaks on this car after buying it, so I know what a vac leak feels like under driving conditions.

I had planned to test further today, but I had other obligations and ran out of time again. I'll answer what I can, but I'll have to come back to them once tested.

- Do you a.) have a functional CEL, and b.) have any stored fault codes?

I don't have the CEL light installed, and I've tried several times to pull codes, but for some reason, I can't call them up at all. I will try again because I would like to see if anything is stored.

- How much vacuum a.) does your engine pull at warm idle, and b.) how much is delivered to the FPR?

I'm waiting for some vacuum fittings to arrive, so I haven't tested the vacuum side of things yet.

- Is the CCV hose the original all rubber, or the newer metal one; and is it totally intact?

The CCV is the metal one. This is one of the first upgrades I did after buying the car, along with replacing all the vacuum lines. I bought the correct hoses that connect the breather to the manifold from Tradition, which match the metal pipe, and ensured the 1mm restrictor was in place. I can confirm the CCV plumbing has no splits. The elbow from the MAF to the throttle body is intact as well. No splits.

- Does your ISV's (possibly old?) hose collapse during hot cranking?]

No, the hose from the front of the manifold to the ISV doesn't collapse during hot cranking. I have two of these hoses, and I have tested both (for good measure). I also have two ISVs that I've tested. They both appear to be fully functional, as far as I can tell.

- What is your battery voltage?

Would need to test, but it's a new battery.

- Is your alternator output steady?

Would need to test. I had the alternator fully refurbished within the past year so I would assume so, but will still test to be thorough.

- Do you have spark while the problem is occurring?

Again, I'd need to test. These kinds of things were on my list, but I've been so focused on the fuel pressure side of things, I haven't got around to exploring other options.

The other thread you linked that mentions the wiring from the ECU to the coolant temp sensor is another one I was going to test. I spotted that in the Bentley manual, but again, I've just been so focused on those fuel pressure readings, I haven't got around to ticking that one off.

I hope even these partial answers will rule out some things. I was hesitant to reply before doing all of the tests, but while it's fresh in my mind, I thought it'd be best to put what I can here for now.