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What other issues can mimic a vacuum leak? #5314638 01/06/20 10:57 AM
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brages Offline OP
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I've got a 1997 Ranger with the 2.3 "Lima" 4-cylinder, and lately it's been giving me fits.

Looking at the fuel trims, it appears to be a vacuum leak (trims are highly positive at idle and even out at higher load and RPM). I've tried to find a leak by several methods and haven't found anything. I'm probably going to replace the intake manifold gaskets (plus, taking off the upper intake will help me get to the spark plugs and PCV).

I'm also thinking about the fuel system... it doesn't hold pressure well after you shut the key off and I suspect the check valve in the pump is bad. I'm thinking of replacing the fuel pump and filter, since I am pretty confident that these parts are bad, but I don't have very high hopes that it will fix it. I checked fuel pressure with the engine running, and it holds pretty well around 30 psi. I would think that fuel delivery would cause the opposite problem as a vacuum leak, though.

MAF... there's a story.. The MAF that was on the truck is overreporting the air from what I can tell. This caused a rich code to be set (P0172, if I remember right). So I put on a new MAF and it seems to be reporting the air correctly, around 2.3g/sec at idle. Worked fine for a few months. But now with a presumed-good MAF, the fuel trims at idle get so high that it goes into open loop mode, where it barely runs. When I put the bad MAF on, the trims are lower -- because it overreports the air flow -- but it stays out of open loop and at sort of runs OK but not well because the fuel trims are going all over the place.

I keep finding threads on the Ranger forums about problems that are a lot like mine and they all go like this:

I have XXXX lean fuel code, any ideas about what it could be?

Sounds like YYYY, replace parts XYZ to fix it.

I replaced XYZ, and it's still doing it!

OK, maybe try this one other part

I replaced that and it's still bad! Anybody?

[End of thread]

There's never a resolution that makes me think, OK it should be this part.

Last edited by brages; 01/06/20 10:58 AM.
Re: What other issues can mimic a vacuum leak? [Re: brages] #5314678 01/06/20 11:34 AM
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Chester11 Offline
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At the risk of just throwing another bad suggestion on the pile - have you been able to take a look at the hose going to the PCV?

On the mazda version - it is buried deep behind the intake. Many seem to report this hose cracking and causing a bad vacuum leak. I was never confident I could induce enough flammable vapor to the area and conduct a good test.

Re: What other issues can mimic a vacuum leak? [Re: brages] #5314685 01/06/20 11:40 AM
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brages Offline OP
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Haven't been able to get down in that area at all... if that's a common issue, that could be it... maybe... I prayed and sprayed with the carb cleaner and it didn't seem to do anything.

I'm thinking going on a gasket-swapping adventure with the intake manifold would give me that chance to get up close to the PCV and its hose -- good idea!

Re: What other issues can mimic a vacuum leak? [Re: brages] #5314700 01/06/20 11:56 AM
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LvR Offline
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Vacuum leak will not massively affect mixture above idle imo ............. Id say its not vacuum leak responsible for your miseries and with the constant high fuel trims would want to have a look at the injectors (possibly blocked/clogged) - that is after you have made sure you can retain a good fuel rail pressure after ignition switch off for many minutes .............. if you have a constant low fuel pressure it can cause exactly the same issues

Re: What other issues can mimic a vacuum leak? [Re: brages] #5314772 01/06/20 12:48 PM
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CR94 Offline
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From actual experiences with my (carbureted) Mazda:
1. Small hidden vacuum hose blown off due to backfire due to snow intrusion due to shielding damage due to road junk;
2. Leaky EGR valve;
3. Secondary throttle not fully closing.


2011 Toyota Prius now at 110K
1981 Mazda GLC (323) retired at 606K
1972 Subaru DL retired at 190K
1954 Chevrolet retired at 121K
Re: What other issues can mimic a vacuum leak? [Re: brages] #5314792 01/06/20 01:10 PM
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Miller88 Offline
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Take a look at the DFPE sensor. I had a bad one on my Taurus for a few years that caused a lot of pinging and general weird issues that pointed to some sort of massive leak. Apparently the sensor was bad causing it to have the EGR valve open a lot more than it needed to be.


18 Forester 2.5I 6M
00 Jeep XJ
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Re: What other issues can mimic a vacuum leak? [Re: brages] #5314817 01/06/20 01:31 PM
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Farnsworth Offline
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If an intake boot ,the large rubber connectors, has a crack after the MAF it causes those kind of problems.

Re: What other issues can mimic a vacuum leak? [Re: brages] #5314891 01/06/20 02:40 PM
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Audios Offline
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Start with a smoke test if there is no vacuum at the oil cap at idle. You can also have bad oxygen sensors, or weak fuel pressure like you said. Clogged cats can throw these faults as well, but very rare. Fuel pressure spec is 35-40 so if youre only making 30 that will definitely have something to do with it.


2002 VW Golf TDI-T5 10/30
2007 Mercedes GL320 CDI-T6 5/40
2005 BMW M3 BMW-Twin Power 10/60
2003 BMW M5-M1 15/50
1998 BMW M3/4/5-M1 10/40
Re: What other issues can mimic a vacuum leak? [Re: brages] #5315270 01/06/20 09:33 PM
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P10crew Offline
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I would look close at your crankcase ventilation system.

Re: What other issues can mimic a vacuum leak? [Re: brages] #5315289 01/06/20 09:58 PM
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Originally Posted by brages

Haven't been able to get down in that area at all... if that's a common issue, that could be it... maybe... I prayed and sprayed with the carb cleaner and it didn't seem to do anything.

I'm thinking going on a gasket-swapping adventure with the intake manifold would give me that chance to get up close to the PCV and its hose -- good idea!

I prefer to use propane to look for a vacuum leak and I use my OBD2 to confirm a bump in idle/change in fuel trim unless the change in engine sound from the propane enrichment is obvious. And remember vacuum hoses are most likely to crack at bends and connection points, so pay special attention to those locations. If it's not a leak, it's more than likely the fuel delivery/injector (s). It's just a process of elimination..


Last edited by Mad_Hatter; 01/06/20 10:00 PM.
Re: What other issues can mimic a vacuum leak? [Re: brages] #5315329 01/06/20 11:04 PM
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Check the BARO reading/PID on your scan tool.....The BARO reading at seal level should be 159 Hz. The reading will go down 3 Hz for every 1000 ft increase in elevation.
This "Leared Value" does require 3 consecutive WOT pulls to be accurate! Let's say the KAM got reset....This would skew the value.

Originally Posted by brages

Looking at the fuel trims, it appears to be a vacuum leak (trims are highly positive at idle and even out at higher load and RPM).


If I'm reading this correctly....Your fuel trims CORRECT under load?, That would be a vacuum leak 95% of the time. But check the BARO PID as it's a good tool for diagnosing MAF sensor issues!


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: What other issues can mimic a vacuum leak? [Re: clinebarger] #5315331 01/06/20 11:11 PM
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Originally Posted by clinebarger

Originally Posted by brages

Looking at the fuel trims, it appears to be a vacuum leak (trims are highly positive at idle and even out at higher load and RPM).


If I'm reading this correctly....Your fuel trims CORRECT under load?, That would be a vacuum leak 95% of the time. But check the BARO PID as it's a good tool for diagnosing MAF sensor issues!


Agreed - I took it to mean that the trims are high throughout and like you wondered about the fact but anyway with no feedback we can only post volumes ..........

Re: What other issues can mimic a vacuum leak? [Re: brages] #5315657 01/07/20 10:37 AM
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brages Offline OP
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Right... with the good MAF, the fuel trims at idle are highly positive (so positive that they send the car into open loop), but at higher rpm/load the fuel trims come down closer to zero. So it sounds like a vacuum leak.

The bad MAF overreports air flow, so the fuel trims are actually slightly negative at idle. Then at RPM they go more negative. Then when the car returns to idle, fuel trims spike positive and it stumbles, then recovers and fuel trims go back to slightly negative. But I don't want to read too much into behavior with the bad MAF.

I'll try to look at the BARO readings to see if I'm right about "good" vs "bad" MAF.

I'm loading up the "parts cannon" with a Rockauto order... planning on replacing the fuel pump, PCV, spark plugs, injector o-rings, and all intake manifold gaskets. Seems like a lot, but the parts are cheap, and getting the upper manifold off will give me good access to the LHS plugs and PCV valve, and let me check out the EGR valve and make sure everything is looking good, at least. I don't have high hopes that this will actually fix anything, but who knows? While I'm changing plugs, I suppose I'll do a compression test.

One thing I have found is that it's somewhat common for high-mile Rangers to have recessed valve seats and this leads -- apparently -- to rough idling but good high-rpm running. However, the owners who had recessed valves didn't see crazy fuel trims. So I'm going to try to sort out my fuel trims first before pulling off the head...

Re: What other issues can mimic a vacuum leak? [Re: brages] #5316280 01/08/20 01:43 AM
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clinebarger Offline
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I believe 30 psi with full manifold vacuum on the FPR is within spec, If the pressure rises & maintains at WOT.....I wouldn't condemn the fuel pump.

Not maintaining residual pressure is a common problem & would condemn A LOT of otherwise good fuel pumps, I personally don't use this to sell or recommend pumps to customers.
IF I need residual pressure to perform Injector Balance Flow Tests.....I'll use a Pressure Vessel so I don't have to rely on the vehicles fuel pump.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: What other issues can mimic a vacuum leak? [Re: brages] #5316518 01/08/20 09:26 AM
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brages Offline OP
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The fuel pump has also had an issue where you needed to prime for a couple of seconds with the key on, or the engine would start, then immediately die. So at least, hopefully, replacing the pump will eliminate that annoyance.

But it seems to me that fuel delivery issues would result in low power at speed and under load, and I haven't noticed any of that. So I don't have high hopes that it's going to fix things.

Another thing that happened recently, I've been getting a CEL for misfire on cylinder #3. I think this is tied to the rough running at idle. Which I still think is vacuum-related. But when I do the plugs I will check compression and especially look at #3 for problems.

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