Severe hesitation on acceleration when coming off idle

Schumi 1 · Post by **Schumi 1** » 21 Oct 2011, 01:23

Hello all,

The severe hesitation remains and I have yet to solve this mystery. Starting a new thread.
I have learned much about the b230f and still I am vexed. As a refresh to new readers here is the problem with new updated information as of today.

1. First start in the AM on cold engine, - very difficult, battery,starter,ignition all ok, have to "pump" the throttle to get it to turn over.
2. After this first cold start, the idle is rough , and seems to be rythmically surging a bit (lumping?) for about a minute or two. then it stabilizes and is smooth (with an induction tach on the plug wire - it reads 780 rpm).
3. warm engine runs fine with maybe a little loss of power on hills or hard acceleration, like it is running on 3 cylinders , but doesn't sound or feel like its missing.
4. Once the engine is hot after an extended period of "stop and go" traffic and/or an hour or so on the highway the hesitation from rolling stop or stop coming off idle is severe (as if it was going to stall out) and if I give it full throttle I can induce a small muffled backfire at the exhaust.

Most recent tests :

1 Fuel Pump , both electric and pressure. First I spliced in a voltmeter at the wire harness to the fuel pump in the back. While driving, and during a hesitation occurence I monitored the meter to look for voltage drop. Nope, steady at around 12.47v at all times. This tells me that I am not losing power to the pump from the relay or bad wire.
On to pressure - I finally was able to hook up a long fuel pressure gauge to the inline schrader valve between the fuel filter and tank. The gauge read 32psi steady at idle and while driving. After many turns around the block, the brick finally hesitated severely and while it was "bogging" the needle on the gauge barely moved if any maybe just to 31psi for a second. It remained at 32psi while driving and during hesitation events. I would think that it would drop significantly on the gauge if the pump was cutting out , unless it is holding pressure behind the filter and losing ahead of the filter ? Only way to know is to drive while it is connected at the rail schrader, my gauge won't fit because it does not have 90deg end fitting on the hose.

Recent Ideas:

1.Losing compression in 1 or more cylinders - misfire when engine is coming off idle or sticking valve or broken spring causing valve not closing correctly.
2.Losing spark ( coil, wires, cap/rotor, plugs - recently inspected , all new). maybe voltage drop / power loss to coil itself, why only from stop ? not on highway or at speed.
3. voltage drop at injectors (not firing) , I tested the resistance on the injector terminals and they all were in spec and identical. perhaps the wires to the injectors are fault when hot. again why only from off idle acceleration ? is there a way to test this inline or at the ECU ?

4. ODB codes are 1-1-1 for both fuel and ignition , if there was HT problem it would set a code yes ?
5. O2 sensor tested again and it is functioning properly.
6. No gas in oil, no oil in coolant, no coolant in oil, no drop in coolant or oil level - head gasket - ok
7.RSR - new , no change
8. TPS - new , tested for continuity - throttle linkage and idle adjustment correct.
9. IAC valve swapped, no change
10. ECU swapped, no change
11. 99% sure no vacuum leak anywhere.
12. ECT, Air Temp, MAP , knock - sensors new and no missing signal OBD codes.
13. fuel pump relay and fuses - ok (above voltmeter test rules out problem.)

I know my exhaust manifold gaskets need replacing but the leak is not that bad yet. hesitation has been there from before they were leaking.

Kirby J · Post by **Kirby J** » 22 Oct 2011, 13:33

I also have a 93 945, (rex-regina) and went through a couple of similar problems. My fuel pump was running, but it was not providing full pressure. I had been told that the fuel pump in this thing was either good or bad, that it did not fail "soft". I found out differently. After replacing nearly everything else I could think of, I changed the fuel pump. No more hesitation. I am not sure, but I think 32lbs of fuel pressure is a bit light. Some of the experts on this sight can tell you the correct FP .
Another cause for low fuel pressure in this car can be found in the gas tank. There is a small device that evens out the flow of gas from the pump. It is about 2 inches long, and looks a little like the brake master cylinder resevoir on an old ford. This will develop leaks, and all your pressure is simply bled off into the gas tank. The fix is simple. remove the device, and replace it with a 2 inch piece of fuel line and two hose clamps. 240K miles later and it has not hurt my car to run without it.

Kirby J
93 945, 417K

Schumi 1 · Post by **Schumi 1** » 23 Oct 2011, 00:23

Thanks for the info, I have read that 32psi is what it should be with the FPR connected. I assumed since it did not drop with a gauge connected during an actual moment of hesitation, the line is not losing pressure. I replaced the Air intake manifold gasket today, just trying another possibility. I'll post if there is any change. Next step , compression test.

Post by **rgk** » 23 Oct 2011, 19:02

Very interesting post. Nice to hear about people splicing in voltmeters, hooking up pressure gauges, to monitor levels while driving and such.

Schumi 1 · Post by **Schumi 1** » 23 Oct 2011, 23:19

Update: I had a new air intake manifold gasket that I hadn't installed because all of my leak tests did not indicate a leaking manifold. I installed it anyway yesterday and test drove it today, unfortunately It did not solve the hesitation problem. I did some research about the torque specs on the manifold nuts, 15ft/lbs seems to be what I found. The two nuts in the center bottom cannot be reached with a torque wrench or any other socket I can imagine, so I had to use a box wrench and give them a good guess. The others I first set to 15ft/lbs but it seemed not very snug, so I bumped them up to 18ft/lbs. I did not notice anything unusual about the old gasket apart from being a bit hardened. I did notice some oil coking on the intake runners and also some light oil on the block side of the gasket. Oh well, it was worth a try. I think the oil is from blow by. I will update if anything else comes up..

Schumi 1 · Post by **Schumi 1** » 11 Nov 2011, 09:12

Kirby J wrote:I also have a 93 945, (rex-regina) and went through a couple of similar problems. My fuel pump was running, but it was not providing full pressure. I had been told that the fuel pump in this thing was either good or bad, that it did not fail "soft". I found out differently. After replacing nearly everything else I could think of, I changed the fuel pump. No more hesitation. I am not sure, but I think 32lbs of fuel pressure is a bit light. Some of the experts on this sight can tell you the correct FP .
Another cause for low fuel pressure in this car can be found in the gas tank. There is a small device that evens out the flow of gas from the pump. It is about 2 inches long, and looks a little like the brake master cylinder resevoir on an old ford. This will develop leaks, and all your pressure is simply bled off into the gas tank. The fix is simple. remove the device, and replace it with a 2 inch piece of fuel line and two hose clamps. 240K miles later and it has not hurt my car to run without it.

Kirby J
93 945, 417K

Hello Kirby J, I am pretty sure you were right on with my problem, after much debate and testing, I pulled the whole sending unit and replaced with new OEM unit that had the pump pre-installed. My hesitation problem is fixed ! Since there was no "visible" sign of wear or broken parts on the old sending unit, and the pump in that one was installed new only 2 years ago, I can only assume this "pulse damper" thing was to blame.
How I figured it out - Pinched off the return line after the rail and measured pressure . The pressure did not increase beyond 30ish psi. I am going to post a "answered" post to my long hesitation post and explain everything so others can share.
I did not break open the little thing but I think you were right as everything else looked tight.

I hope my 93 940 didn't suffer too much from the "lean" running all the time....

Best,
Schumi 1

Kirby J · Post by **Kirby J** » 11 Nov 2011, 16:09

Just curious, what does your fuel pressure run now?

Kirby J

Schumi 1 · Post by **Schumi 1** » 11 Nov 2011, 17:40

32 +/- with the FPR connected, 44ish disconnected. It had to be bleeding off back into the tank somewhere and that section of fuel hose attached to the sending unit was new and the clamps were solid.. The other interesting thing is this one sounds a bit louder when you start it up...
The Regina system does not use a check valve to maintain fuel pressure at shutdown. I suppose the pump could have been "weak" but that doesn't really make sense to me if it was only 2 years old and the sock looked clean. Anyway I am glad it is fixed..
I still have a bit of rough idle when the fan kicks on or at first start, I think I need to clean the IAC and throttle body. Definately no more hesitation...