Itinerant Air-Cooled

Late last summer bought a 95 pontiac bonneville for my step daughter as her first car. Has the venerable 3.8 V6 with about 110k miles. Has a hard start problem that I am at my wits end trying to solve. After it is thoroughly warmed up ... say 30 min of driving around... it will start fine the first 15-20 minutes after you turn it off. But between 20min and 45-50 min if you try to start it, it cranks like crazy but won't fire. but after it sits for 30 min or so, starts fine. It is EXTREMELY consistent about the timing and not starting. If it was carburated I would think vapor lock, but that obviously isn't the case. It's been plaguing me for 3 - 4 months now. I've posted on a board specific to this model, and pretty much have tried / replaced everything recommended...no dice.

This what has been done:

- swapped coil packs
- swapped ignition control module
- new idle air control valve
- cleaned throttle position sensor and MAF sensor
- replaced upper and lower intake manifold gaskets as well as new upper manifold and PCV valve
- new crank position sensor
- new T-stat
- new coolant temp sensor
- new fuel pressure regulator
- new fuel filter
- blew out air filter (looked pretty new)
- flush n fill radiator
- new plugs

Some more stats:
- fuel pressure while running is a rock solid 34 psi - no wavering. creeps up to 40 if it sits, so don't think it is a bleed down issue.
- vac is 18" ....little low, but again rock solid
- When you get close to the witching time of 15 minutes it will be hard to start and will 'chug' with an oscillating idle for about 30-60 seconds and then it 'clears its throat'. almost like it is running rich??
- oil pressure is a low (about 15 psi at idle) unless i run a qt of lucas oil in it - that brings right back up to spec.

Obviously it is heat / heat sink related. I'm at a loss... engine needs spark, gas, and compression in the right sequence to start

Does the check engine light come on?

How about the crank positioning sensor? (of course I don't even know if it has one).

When it's not starting is the time to diagnose this. It's easy to provide some fuel, just pull off a vacuum hose on the intake and spray some engine starting fluid in the hole. It it still doesnt start you know you have a spark problem. it it does start fuel.

Grasping at straws here: intake air temperature sensor?

- replaced the crank position sensor (yup, has one). And no check engine light.

- replaced the idle air control valve ... if that's what is meant by the air temp sensor... didn't replace the MAF.

- good call on squirting starting fluid down a vac line ... however, that will only work if it is running lean...not rich :) It's acting like it is running rich...if i'm just over the witching time and it acts like it wants to start, if i give it partial throttle, or even WOT, all positions seem to make it worse.

I threw in the towel and dropped it at my trusted local indy tonight.

Took it to a local indy mechanic. Said it was an O2 sensor and the fuel pump (pressure was reading low...which I knew; about 34 psi) to the tune of $860. O2 sensor made sense.. I could see heat sink issues effecting this. Fuel pump didn't; why would it pump fine 15 min after shut - off, then not pump fine for the next 30 min, then pump fine after that!?

I asked if he could be certain this would fix the problem, and he said no. I only had him replace the O2 sensor.... we will see.

He wanted nearly $900 to do those 2 jobs? Thats completely outrageous. The fuel pump is about 3 hours and the egr maybe 1.

TrollFromDownBelow wrote:Took it to a local indy mechanic. Said it was an O2 sensor and the fuel pump (pressure was reading low...which I knew; about 34 psi) to the tune of $860. O2 sensor made sense.. I could see heat sink issues effecting this. Fuel pump didn't; why would it pump fine 15 min after shut - off, then not pump fine for the next 30 min, then pump fine after that!?

I asked if he could be certain this would fix the problem, and he said no. I only had him replace the O2 sensor.... we will see.

Heat sink issues come in two flavors:
LEAN
leaking check valves cause boiling in the lines - starvation after about five to 30 minutes of sitting
fuel pump has one, pressure regulator has the other

RICH
a leaking cold start injector will flood the intake with a pool of gasoline and individual injector leak is much less likely to cause a global starting issue

02 sensor has nothing to do with a transient starting problem. If it is bad, it affects engine operation across the operating range.
Colin

Amskeptic wrote:

TrollFromDownBelow wrote:Took it to a local indy mechanic. Said it was an O2 sensor and the fuel pump (pressure was reading low...which I knew; about 34 psi) to the tune of $860. O2 sensor made sense.. I could see heat sink issues effecting this. Fuel pump didn't; why would it pump fine 15 min after shut - off, then not pump fine for the next 30 min, then pump fine after that!?

I asked if he could be certain this would fix the problem, and he said no. I only had him replace the O2 sensor.... we will see.

Heat sink issues come in two flavors:
LEAN
leaking check valves cause boiling in the lines - starvation after about five to 30 minutes of sitting
fuel pump has one, pressure regulator has the other

RICH
a leaking cold start injector will flood the intake with a pool of gasoline and individual injector leak is much less likely to cause a global starting issue

02 sensor has nothing to do with a transient starting problem. If it is bad, it affects engine operation across the operating range.
Colin

Not sure if it has a cold start injector....nothing I've read has made mention of it. It does have an idle air control valve, which I've replaced. I'm leaning towards LEAN, specifically the fuel pump based upon low PSI readings ...runs at 34 supposed to be at 40. Fuel pump is the likely culprit. Also, I replaced the fuel pressure regulator already.

It does idle smoother with the new O2 sensor. Just started it up after letting it sit for 20 minutes. It DID start, although had a slight bobble after it fired. Although it normally would not start, or start but run really rough for 20 seconds so this would be a big improvement, however, I'm not convinced it is fixed. It may be a combo of O2 + weak FP. But even if the O2 bought me a month or so, that's good.

If it happens again, check the fuel pressure when it's not starting. I'd even check the fuel pump electrical. I know Hondas from the early 90's would do that same thing due to cracking solder in the main fuel relay.

Also, sniff the tail pipe when it's doing so. If it's flooding, you'll smell it.

Based on what I've read (here included), and what have been told by other ASE mechanics... as well as process of elimination I believe it is a bad fuel pump. It IS low on pressure (checked this myself). I'm going to do the job myself though ... only $100 for a brand new oem A/C delco fuel pump vs $550-$650 for someone else to do it. That's next weekend's project.

TrollFromDownBelow wrote:Based on what I've read (here included), and what have been told by other ASE mechanics... as well as process of elimination I believe it is a bad fuel pump. It IS low on pressure (checked this myself). I'm going to do the job myself though ... only $100 for a brand new oem A/C delco fuel pump vs $550-$650 for someone else to do it. That's next weekend's project.

You'll have fun.

I did a BMW 540 fuel pump at a Chevron station at Tapahatchipeiwhatchamacallit Summit in 2007, with a totally full tank of gas just having been put in to stave off my own worsening fuel starvation issues.

When you think of what that depth of petroleum liquid can do to propel a 3,700 lb car 250 miles!!, you put out your cigarette as you lift the pump out of the tank.
Colin

Amskeptic wrote: When you think of what that depth of petroleum liquid can do to propel a 3,700 lb car 250 miles!!, you put out your cigarette as you lift the pump out of the tank.

And you use non-ferrous tools to loosen and tighten the pump lock-ring, because you don't want any chance of sparking at all, no no no.

Wait, a whole 250 miles on one tank? Boy, that is a tank!